Initial test for middleware/middleware.go

Fixes bug introduced in 0ac8bf58 - Also note that setup functions no longer have access to server port. Will need to fix later.

log, errors: Support system log (except Windows)

Specify log file of 'syslog' to output to system log

Also change sha1 to md5 for the directory scans; slightly faster.

See github.com/russross/blackfriday/issues/189

This behavior can still be overridden by bind directive

markdown: generate static sites after links

markdown: Watch for file changes for links. Removed sitegen requirement for link index.

Bug detected by go-fuzz (more fuzzing is required; need larger corpus)

We do this by returning an error and stopping link generation (which scans the markdown path for files)

Even if defined for multiple hosts. Startup or shutdown callbacks registered by any directive (startup, shutdown, markdown, git, log, etc.) will only run as many times as it appears in the Caddyfile, not repeated for each host that shares that server block. Fixing this involved refactoring three packages (yeesh) and we need to restore some tests that are no longer valid (that used to verify splitting a multiServerBlock into multiple serverBlocks).

Powers of 2, or base 1024, rather than powers of 10 or base 1000. The powers of 2 are technically more accurate.

Also, summary truncated at nearest space instead of middle of word, and code spans become part of summary.

markdown: Added test on static files generation

markdown: Add .Links action and flattened metadata structure

.Links available only for generated sites and variables no longer in a [variables] category in metadata (flat structure).