includePrefix & includeExt added

Adds the ability to define a includePrefix (filename prefix), for those whom included files are always prepended by "_", and also a file extension to be appended.

Thus, making possible such include:

/* @include "functions" */

Where it would be before:

/* @include "_functions.js" */

(similar to how SASS does)
Update Readme.md
Merge pull request #11 from valeriansaliou/master

Ability to customize file including rule (prepend/append a string to the filename)