Adds memory file system support (including tests) Closes #160

[hinell]

Rationale

• There is need to have an ability to use alternative file systems (fs). Like memfs for instance.
• The wepback is a good example. It can be configured to use its memory-fs module to generate and keep all its assets in memory to gain better performance compared to hdd. By using the compiler.outputFileSystem api as a source we can serve generated files directly from memory.

Changes

• Options

send(..., ..., { fs: hddFileSystem })

Tests

• Adds basic tests by reusing old ones (It was ad-hoc solution. Let me know if there is someone who can assist me in writing better tests )
• Adds command for testing memfs instance (test:m where m stands for in-memory fs)
• I just added very basic tests which checks out whether fs has some properties necessary to normal functioning. It is obviously only shallow checking and won't save you from having errors arising from using incompatible file systems

Caveats

• You can't use webpack's memory-fs (the version 0.4.1) currently due to lack of some interfaces but you can use memfs instead. The configuration is tricky though, cause you will have to manually assign a .join() method to the fs instance.