Add external option to allow whitelisting external dependencies

[anandthakker]

This adds the external: 'globpattern' option, which can be used to whitelist external dependencies. Glob matching is relative to the node_modules folders adjacent to each index, so the glob pattern external should match node_modules/external, and external/node_modules/deep should match node_modules/external/node_modules/deep.

[anandthakker]

Issue: #56

[anandthakker]

@tmcw I think this is pretty much ready.

[tmcw]

@anandthakker are tests passing on your side? Getting a test failure with

> documentation@1.0.6 test /Users/tmcw/src/documentation
> eslint index.js test/*.js test/streams/*.js && prova test/*.js test/streams/*.js


/Users/tmcw/src/documentation/lib/module-filters.js:36
          if (pkg.main) {
                 ^
TypeError: Cannot read property 'main' of undefined
    at matchGlob (/Users/tmcw/src/documentation/lib/module-filters.js:36:18)
    at /Users/tmcw/src/documentation/lib/module-filters.js:53:18
    at Array.map (native)
    at Object.postFilter (/Users/tmcw/src/documentation/lib/module-filters.js:52:10)
    at /Users/tmcw/src/documentation/node_modules/module-deps/index.js:320:38
    at onresolve (/Users/tmcw/src/documentation/node_modules/module-deps/index.js:159:25)
    at /Users/tmcw/src/documentation/node_modules/module-deps/node_modules/browser-resolve/index.js:250:24
    at /Users/tmcw/src/documentation/node_modules/module-deps/node_modules/resolve/lib/async.js:46:14
    at process (/Users/tmcw/src/documentation/node_modules/module-deps/node_modules/resolve/lib/async.js:173:43)
    at ondir (/Users/tmcw/src/documentation/node_modules/module-deps/node_modules/resolve/lib/async.js:188:17)

[anandthakker]

@tmcw hmm, strange -- they're passing on my end...

[anandthakker]

@tmcw note: just rebased.

Still can't reproduce the test failure, and I'm puzzled at the one you're getting. Can you update module-deps to 3.7.8 and try again?

[tmcw]

Yep, I think all my deps are up to date, but somehow still getting this issue https://gist.github.com/tmcw/91093bf851f52dc540ae digging into it now...

[tmcw]

@anandthakker do you have an uncommitted test/fixture/node_modules/external2 directory possibly?

[anandthakker]

Aw, crap. Yeah, that was it -- got tripped up by my own global .gitignore rule. Sorry about that!

[tmcw]

👍 great - I'll merge and then add a test to make sure that missing modules aren't crashers