SourceMap#fromSource should preserve columns

This fixes plumber losing column information during certain operations.

Fixes https://github.com/plumberjs/mercator/issues/5
Yeah, the test would have to scan the file with esprima itself and match on a token by token basis, maybe re-using bits of the source map API to work out what the data is. Essentially you'd be repeating the tokenisation process into the test, which would be a bit silly. So I think it's better like this. Kinda.