Show context when printing warnings

[JanStevens]

Hello,

When using include tags the warnings don't reference the included file but rather the concated result. For example my main api has just 4 include tags and I have a warning on line 504. I now have no clue where to look.

I've created a PR for the gulp-aglio package that does include the context of the error. I think this should be added in the main Aglio package, an example:

I think this could be done easily in the bin file, optionally It would be better if we had a reference to the original source file.

Related to: #96 but this might be a temp solution?

Regards,

[danielgtaylor]

Love it! We should definitely add this.