Adds nested directory output using option --inputContext - #31

If input is **/*.css, this works nicely. However if it is something like src/window/**/*.css (which should match multiple files) there seems to be a problem with the resulting value of input.

      inputPath = input.replace( argv.inputContext, '' );
                       ^

TypeError: Cannot read property 'replace' of undefined
    at compile (/home/mike/projects/mixlac2/node_modules/postcss-cli/index.js:183:24)
    at arrayEach (/home/mike/projects/mixlac2/node_modules/neo-async/lib/async.js:3468:9)
    at Object.each (/home/mike/projects/mixlac2/node_modules/neo-async/lib/async.js:4178:9)
    at Object.<anonymous> (/home/mike/projects/mixlac2/node_modules/postcss-cli/index.js:147:7)

Sorry, ignore that. I wasn't understanding the meaning of context.

@vbalcom There is a typo in the commit message for 5fc007e6d6bab:

output dir will represent nested folder structures found from a blobbed input.

Perhaps this should be squashed into one or two commits.

@RyanZim Agreed, I'll spruce this up and re-submit a pull request this evening.

If you can figure out what's causing the tests to fail, that would be great.

This looks like the missing feature I was looking for in the CLI.
Is there any chance that this could be merged soon?
@RyanZim It looks like the test run through now.

@Hirse We are currently in the middle of a full rewrite of the codebase for v3. This feature will have to be totally reimplemented for the v3 codebase. I haven't closed this PR as a reminder to consider porting this to v3+

Do you have an estimated release date for v3?
If it will still take a while, it would be great to merge this for v2 anyway.

I would really love to have this feature, as my decision to use postcss for an Angular 2 project (which has distributed CSS in the folder hierarchy) depends on it.

@Hirse I hate to set a date for it, but you can track progress here: https://github.com/postcss/postcss-cli/projects/1 & https://github.com/postcss/postcss-cli/milestone/1. Probably early February, though.

Any news on this? I would really love to use this feature.

@mstoltenburg We need to add this ability, and --inputContext is the best proposal I've seen so far. PR welcome to reimplement this for the v3 codebase.

Superseded by #121