feat(cli): inline whole directories

[cossssmin]

Closes #337, closes #374

Note: builds on #668 (CLI test coverage), so this shows that commit too until #668 is merged.

The CLI can now inline a whole directory:

juice src dist

It's detected automatically when the input is a directory (no new flag). Every .html and .htm file in it, including subdirectories, is inlined and written to the same relative path in the output directory.

• all CLI options apply to every file (--css, --options-file, --web-resources-*)
• relative <link>s resolve against each file's own directory
• if the output directory is inside the input one (juice src src/dist), it's skipped, so outputs never get re-inlined on the next run
• if a file fails, it's reported with its path, the rest are still processed, and the CLI exits with 1 at the end
• a directory input without an output directory is an error, and - (stdin/stdout) can't be combined with directories

One thing worth pointing out: juiceFile fills in webResources.relativeTo on the options object it's given, only if it isn't set yet. Sharing one options object across files would make every file resolve its links against the first file's directory, so each file gets its own copy (there's a test with nested <link>s that fails without it).

Performance

200 files (12KB each, remote resources off), same output:

	Time
juice src dist	842ms
find src -name '*.html' | xargs -P 16 juice ... (the workaround from #374)	3,046ms

Each separate juice process pays around half a second of Node startup + imports, so doing it in one process is ~3.6x faster, even with 16 processes in parallel on a 24-core machine.

Also documented in the README's CLI section, and the --help argument descriptions mention directories.

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