Improve custom importer to pass parent import context

One example would be a custom importer that loads the
content from the web. If this includes another import,
it should be loaded relative to the parent file. To do
this, the custom importer needs to know the context of
the parent import. Which is what this commit will fix!
Don't delay String_Constant's that are name colours

This will allow the eval visitor to correctly coerce the string node
to a color node.
Merge pull request #695 from xzyfer/fix/map-keys-as-colors

Fix colours as map keys
keywords() should gracefully fail for a non variable arglists
Merge pull request #696 from xzyfer/fix/keywords-without-keyword-args

keywords() should gracefully fail for a non variable arglists
Implement negative '@for' loops (Fix #703)
Implement new colors (rebeccapurple and transparent)

Fix https://github.com/sass/libsass/issues/699
Fix https://github.com/sass/libsass/issues/700
Implement inspect function (Fix #701)

This also implements missing Map Inspect Operator!
Implement feature-exists()
Merge pull request #705 from mgreter/fix/negative-for-loop

Implement negative '@for' loops (Fix #703)
Merge pull request #706 from mgreter/feature/inspect-function

Implement inspect function (Fix #701)
Merge pull request #707 from mgreter/feature/new-colors

Implement new colors (rebeccapurple and transparent)
Merge pull request #709 from xzyfer/feat/feature-exists

Implement feature-exists()
Fix default argument passing for custom c functions

In reference to https://github.com/sass/libsass/issues/708
Fix fractional output for truncated zeros
Implement @error function and Sass_Warning for C-API
Code: Adds an include statement for VS.
Merge pull request #719 from am11/master

Code: Adds an include statement for VS
Fix embedded sourcemap content type
Merge pull request #723 from xdissent/map-content-type

Fix embedded sourcemap content type