faster_lsi: Massively accelerate LSI performance.

Currently, Classifier::LSI rebuilds the index every time an entry is added.  This runs into massive performance overheads on my website; theoretically, disabling automatic index rebuilds, and explicitly rebuilding the LSI index at the end of the LSI repopulation should speed things up nicely.

As a side note, here, I use pandoc-ruby to provide a more featureful Markdown transformer, so be mindful that the numbers I quote here have artifically imposed I/O overheads.

With just the 76 posts I wrote this year (abysmal, I know), I come up with the following figures:

    Without faster_lsi:
      jekyll --lsi  16.91s user 0.88s system 97% cpu 18.302 total
    With faster_lsi:
      jekyll --lsi  2.72s user 0.77s system 88% cpu 3.940 total

With 109 posts, we begin to see even better improvements:

    Without faster_lsi:
      jekyll --lsi  51.00s user 1.47s system 98% cpu 53.060 total
    With faster_lsi:
      jekyll --lsi  5.04s user 1.12s system 91% cpu 6.735 total

At this point, we begin to see I/O overheads being slower than LSI when faster_lsi is active.  I call that fairly conclusive.  But wait, there's more.  I have 273 posts lying around... I wonder what happens if I feed them all in.  With faster_lsi, it was nice and clippy. Without it, I simply gave up, and went and refilled my cup of tea. And it was still going.

    Without faster_lsi:
      jekyll --lsi  1277.86s user 10.90s system 99% cpu 21:30.29 total
    With faster_lsi:
      jekyll --lsi  34.62s user 4.43s system 96% cpu 40.430 total

That is, in anyone's books, a major improvement.  Note, however, that I don't know just how well this will perform with `jekyll --auto` because I don't know how it does the LSI rebuilds.  I _think_ (but please, don't commit me on this) that the LSI is rebuilt every time Jekyll picks up a file change.

So, all up, the performance improvement is massive, and scales depending on how many files you have.  At the last point, the improvement is just on 3200%.

A more optimal solution would be to cache the LSI index and/or content data somehow.  I'll leave that to when faster_lsi takes over ten minutes to run.