Rename reference of multi-file to directory

[xzyfer]

The multi-file terminology is unintuitive and could be seen as suggesting we support multiple input files. This is how our documentation describes the multi-file mode.

Pass a directory as the input to compile multiple files. For example: node-sass sass/ -o css/. The --output flag is required and must be a directory when using multi-file compilation.

IMHO we should stop conflating these two terms and standardise on directory. I think this is much clearer.

Thoughts @am11

[am11]

This is indeed the directory compile.
The multi-file compile would be where user will provide paths from varied roots (perhaps using glob's 'brace syntax'?), but we don't support glob for input (yet?) :)

[xzyfer]

Cool. Upgrade to feature request.