- change to check existence of strip to avoid not found errors
 - change to use xargs -r option to avoid empty input errors
 - change to chack magic number of files to avoid strip wrong-format errors

Change crew to add CREW_NOT_STRIP constant and strip binaries and libraries unless CREW_NOT_STRIP is defined.

Change crew to add a CREW_LIB_PREFIX constant which refers "/usr/local/lib" or "/usr/local/lib64" depends on ARCH.

  - Update 'crew help search' command to explain improvements

Usage is like below.

```
$ crew search vi
(i) vim: Vim is a highly configurable text editor built to make creating and changing any kind of text very efficient.
(i) vidstab: Transcode video stabilization plugin.
vifm: Vifm is an ncurses based file manager with vi like keybindings/modes/options/commands/configuration, which also borrows some useful ideas from mutt.
(i) libxvid: The free video codec that is strong in compression and quality.
neovim: Neovim is a refactor, and sometimes redactor, in the tradition of Vim (which itself derives from Stevie).
$ crew search vi extra
(i) vim: Vim is a highly configurable text editor built to make creating and changing any kind of text very efficient.
http://www.vim.org/
version 8.0-1
(i) vidstab: Transcode video stabilization plugin.
http://public.hronopik.de/vid.stab/
version 0.98b
vifm: Vifm is an ncurses based file manager with vi like keybindings/modes/options/commands/configuration, which also borrows some useful ideas from mutt.
https://vifm.info/
version 0.8.2
(i) libxvid: The free video codec that is strong in compression and quality.
https://www.xvid.com/
version 1.3.4
neovim: Neovim is a refactor, and sometimes redactor, in the tradition of Vim (which itself derives from Stevie).
https://neovim.io/
version 0.1.7
$ crew search ^vim$ extra
(i) vim: Vim is a highly configurable text editor built to make creating and changing any kind of text very efficient.
http://www.vim.org/
version 8.0-1
```

`crew search` use to only return a single result if your query string
matched a package name exactly.

`crew search` now supports regex.

chronos@localhost / $ crew search ^lib
Found libpcap, version 1.8.1
Found libjpeg, version 9.1
Found libunwind, version 1.1
Found libxslt, version 1.1.28
Found libffi, version 3.0.13-1
Found libssh2, version 1.4.3
Found libtool, version 2.4.6
Found libpipeline, version 1.4.1
Found libevent, version 2.0.22
Found libxml2, version 2.9.2
Found libgd, version 2.0.33
Found libtiff, version 4.0.7
Found libpng, version 1.6.26
Found libsigsegv, version 2.10

  - Fix grammar

  - Add colorization
  - Add description and homepage metadata in search results

Change crew to hide `buildessential` from dependencies list since it is a fake package.
Change crew to not check :build dependencies if a target is already installed.
Chagen crew to remove unnecessary push_dependency call.
Change package.rb to update comments

  - Provide information about usage for each command
  - Update README.md to add commands and correct grammar

  - Fix /usr/local/bin/crew:67:in `+': no implicit conversion of nil into
    String (TypeError) with 'crew build', 'crew install' and 'crew install'
    commands if no package is provided.
  - Fix notice 'Package  isn't installed.' with 'crew remove' if no package is provided.

Also modify to use new working_on_source? function

Set XZ_OPT environment varaible for archiving.
Support check method for aggressive test.

multiple packages even there are errors in the middle of compilation.  This solves such problems.