Add "resolve" option to createFilter

[lukastaegert]

This PR will add a new resolve option to createFilter to allow specifying a base directory other than process.cwd() or disable resolution altogether.

Resolves #39
Resolves #20

It is specified via an optional third parameter:
createFilter(includes, excludes, {resolve: '/my/base/dir'})

Passing a string as resolve will resolve patterns like this:
path.resolve(resolve, pattern)
instead of just
path.resolve(pattern)

Passing false will disable this behaviour altogether and directly use the supplied pattern (which is useful if you want to match virtual directories as well).

[eight04]

Can we have some tests against virtual modules?

expect(filter('\0proxy-modules:x')).toBe...
expect(filter('\0proxy-modules:' + path.resolve('x'))).toBe...

[lukastaegert]

I allowed resolve: false to match ids starting with a \0, added a test and also a note to the readme. Note however that using : as a separator in your virtual ids can have an adverse effect on matching (I think minimatch confuses this with windows drive letters or similar), so you should probably use something else as separator and also consider this when putting full paths as substrings into virtual ids on Windows.

[lukastaegert]

Ah, looking into your motivation, maybe I should also fix the names of proxy modules in rollup-plugin-commonjs...