rewrite sourceMappingURL with full path so source maps can be found for ...

...3rd party plugins.

I had a problem since the source map for the Vim plugin was located at /Users/nick/Library/Application Support/LightTable/plugins/Vim/vim_compiled.js.map. The path to the source map in vim_compiled was represented as "sourceMappingURL=vim_compiled.js.map". The problem is that the source map couldn't be found since LT was looking for it relative to the directory in which LT was running. This patch simply rewrites all the source map paths when the js gets eval'd to use the full path of the eval'd js so that they can be found.