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!
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
Merge pull request #710 from mgreter/feature/default-args-cust-fn

Fix default argument passing for custom c functions
Merge pull request #716 from mgreter/fix/fractional-truncated-zeros

Fix fractional output for truncated zeros
Merge pull request #691 from mgreter/feature/importer-parent-context

Improve custom importer to pass parent import context
Merge pull request #715 from mgreter/feature/overload-error-fn

Implement @error function