add an option to specify file extension

[SEIAROTg]

When processing an file without extension name, gulp-sourcemaps will not add comment after the code. However, sometimes we need to deal with some js files without extension names (e.g. bin/www generated by express-generator). In this case, I hope there can be an option to manually specify the extension.

[phated]

I actually think the default to add no comment is wrong. Willing to accept a PR to always add the comment.

[nmccready]

#347 seems related, is this to support alternative extensions or is this more of a file map to assign an extension to a specify a file without one?

Either way I think the code for this could be banged out in the same PR.

[domoritz]

Is there a current workaround to support mjs and cjs files other than renaming files and replacing .js in the generated source maps after the source maps have been generated?