Add opts.expose object to populate resolved pathnames for exposed modules

[jmm]

This is to support related changes on browserify (browserify/browserify/pull/1077).

What's the logic for what properties from opts become this.something vs remaining accessible as this.options.something?

[jmm]

What's the intent of this part?:

if (rec.expose) {
    self.options.modules[rec.expose] = file;
}

Before this pull and browserify/browserify/pull/1077 it means that the following gets the file treated as built in (e.g. transforms aren't run):

b.require('./somefile', {expose: './somefile'});

Whereas these are ok:

b.require('./somefile');
b.require('./somefile', {expose: 'whatever'});

After these pulls it means that this would also get the file treated as built in:

b.require('./somefile');