Use latest Go minor versions for testing

Signed-off-by: Alexander Morozov <lk4d4@docker.com>

markdown: Use less strict condition to avoid problems with concurrency

In latest go versions TestWatcher fails pretty often, because it is
"more concurrent" now. Reproducible with go master:
while go test github.com/mholt/caddy/middleware/markdown; do :; done
Signed-off-by: Alexander Morozov <lk4d4@docker.com>

Fix all vet warnings and add vet check to CI

markdown: fix race in accessing map

Fix race in test

Signed-off-by: Alexander Morozov <lk4d4@docker.com>

Signed-off-by: Alexander Morozov <lk4d4@docker.com>

Signed-off-by: Alexander Morozov <lk4d4@docker.com>

Signed-off-by: Alexander Morozov <lk4d4@docker.com>

Signed-off-by: Alexander Morozov <lk4d4@docker.com>

Signed-off-by: Alexander Morozov <lk4d4@docker.com>

Signed-off-by: Alexander Morozov <lk4d4@docker.com>

markdown: Use markdown.Config as pointer everywhere

* As value mutex was copied and therefore synchronization worked wrong
* It's pretty big structure with reference types, so copying create unnecessary
  pressure on GC
Signed-off-by: Alexander Morozov <lk4d4@docker.com>

basicauth: Fix format call in tests

Signed-off-by: Alexander Morozov <lk4d4@docker.com>

middleware: Complete test coverage for replacer 

core: Configuration as command line arg #222

basicauth: Add htpasswd support

log, errors: Use localtime for the log roller timestamp

Fix formatting directives in tests

Signed-off-by: Alexander Morozov <lk4d4@docker.com>

log, errors: Added lumberjack library for log rolling

The errors and logs can now have log rolling if provided by the user.
The current customisable parameter of it are:
The maximal size of the file before rolling.
The maximal age/time of the file before rolling.
The number of backups to keep.

browse: Initial test for json response in browse.go

If the password arg starts with htpasswd=, then the rest is treated as
the file name of the htpasswd file, and used for md5 and sha1 hashes.