Delete plugin auxiliary metadata from files

[ycherniavskyi]

If found possibility to resolve #18 in minor harm way - just delete auxiliary metadata from files after plugin finishes its work.
So with such fix plugin could be called as many time as needed.

I tried to implement the test for this functionality, but without changing plugins parameter of test function from Object to Array it seems not possible (because this test must use the same plugin .. several times).

[ycherniavskyi]

With this functionality my current usage looks something like this:

...

metalsmith
  .use($m.jstransformer({
    pattern: '**.html.md',
    layoutPattern: '!**',
  }));

metalsmith
  .use($m.branch('blog/*/**')
    .use($m.wordCount())
    .use($m.betterExcerpts())
  );

metalsmith
  .use($m.jstransformer({
    pattern: '**.html*',
    layoutPattern: 'layouts/**',
  }));

...

[RobLoach]

Good call, we don't need that cached information. Nicely done! 👍