Exposing options.basepath as a command-line flag

**This is only a partial pull-request**. I'm not super-familiar with your codebase; no tests have been updated, and I have to admit I don't even know exactly what I'm doing.

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My use-case: I'm instrumenting for coverage with `coffee-coverage`, then generating coverage data with `mocha-lcov-reporter`, and finally sending that information off to [Coveralls](http://coveralls.io). However, I'm running into issues, because my invocation of `coffee-coverage` is omitting the *path* to files:

    ./node_modules/.bin/coffeeCoverage --path=relative --exclude 'additional.coffee' "Source/" "Source/"

In the resulting coverage data, your instrumentation is labeling lines as something like the following:

    _$jscoverage["data.coffee"] = [];

This throws off the `coveralls` gem, as it doesn't know that your library was told only to look at files in `./Source/`, and since there's no path associated with the filename in your instrumented results, it craps itself with a file-not-existing error.

So, long story short: I have no idea why, when I call it as `coffee-coverage --path=relative`, the paths are removed from the instrumented filenames.

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I don't quite understand your codebase, and I'm not sure what the use-case of other users or yourself may be, that the above operation is the desired one … so, instead of trying to change it to leave those paths in, I discovered I can kind of ‘hijack’ `getRelativeFilename()` to ‘fix’ my paths. I found out, accidentally, that when `options.basepath` is set to `"."`, the resolution system will leave a full (with respect to the project), but relative (with respect to the system), path to the file … which is exactly what I need!

To fix it temporarily, I've exposed that `basepath` variable as a flag on the command-line executable. Whether or not you have a preferred way of solving my problem that *isn't* this hack, that might be useful to others *anyway*; I figured it's something nice to have control over.

So: Take a look at this, and let me know if you've got an idea for a better way of solving my relative-paths issue. (=