Feature: Provides sourceMapRoot option

[am11]

Related discussions: #731 & sass/libsass#908.
//cc @itsnydell and @joliss

According to the source-map v3 docs:

(sourceRoot is) An optional source root, useful for relocating source files on a server or removing repeated values in the “sources” entry. This value is prepended to the individual entries in the “source” field.

Further it goes:

Resolving Sources
If the sources are not absolute URLs after prepending of the “sourceRoot”, the sources are resolved relative to the SourceMap (like resolving script src in a html document).

Therefore, node-sass will pass the value of sourceMapRoot (or source-map-root) to the libsass as is, which will return it back in source map as sourceRoot, without affecting anything else.

See the usage example in written tests.

Issue URL: #776.