add sourcemap support for grips.render(..)

[getify]

Not exactly what you're thinking, probably. Adding sourcemap support means that grips.render(..) would be able to accept a sourcemap to map the line/column values reported in any errors back to an original file.

What kind of original file if not the grips template itself? Good question.

grips-css is going to produce standard grips template files from gcss (grips-css) style files. So it'd be very nice if grips.render(..) could map any errors back to the original gcss file instead of the generated grips template file.

Forseeing possible other applications, where tools could auto-produce grips templates from other files, like source HTML files, other templates (aka, template conversion, etc), and the like.

---

It's a separate but related task to have grips compile (and of course, grips-css compile) produce sourcemaps.