if _options.basedir exists, use it in external, exclude and ignore

[tufandevrim]

Not sure if it was intentional or not, main option basedir (this._options.basedir) is being ignored in external, exclude and ignore functions. It is being taken into account in require function by the way https://github.com/substack/node-browserify/blob/master/index.js#L114

[jmm]

Hello @tufandevrim, thanks for contributing. At a glance this does seem to make sense, but it'd help to have failing tests that show the current implementation doing the wrong thing.

(Side note: totally not your fault, but I noticed that these all have an undocumented opts param and read at least opts.basedir from it...ugh.)

[tufandevrim]

Hi @jmm
Please take a look at the test I just added and let me know if it makes sense or not.
If you run this test without the changes proposed in this PR, it will fail unless you add basedir option explicitly to this line https://github.com/substack/node-browserify/pull/1551/files#diff-f101fe74380a155175b7b9240f1b1fcaR15

[jmm]

Thanks @tufandevrim! I or someone will take a look when there's a chance.

[pushred]

This threw me off today. Because of the undocumented opts param on ignore I assumed that this._options was the one to use.. I'd submit a PR for a doc update but seems this is the better approach if require already does this.