UglifyJS sometimes uses `e.msg` for errors. Fixes #51
Add tests for `preserveComments` option.
That was some strange formatting [skip ci]
Refactor out minify try/catch and options parsing
Use vinyl-sourcemap-apply to merge source maps. Fixes #43

I still have to pass the current source map to UglifyJS, to generate
a useful `sources` property on the source map.