Speed up HTML encoder for 1.2

[korny]

• create less objects during setup
• cache most recent 100 escaped strings
• don't extend output unless necessary
• lazy CSS loading and parsing

Quick benchmarking yields a ~10% speedup for Ruby code:

2.3.0@coderay ~/ruby/coderay:master rake bench
/Users/murphy/.rvm/rubies/ruby-2.3.0/bin/ruby bench/bench.rb ruby html 3000
encoding 3000 kB of ruby code to html...
run 1:  1.69 s, 1774 kB/s
run 2:  1.67 s, 1794 kB/s
run 3:  1.68 s, 1786 kB/s
run 4:  1.66 s, 1805 kB/s
run 5:  1.69 s, 1775 kB/s

2.3.0@coderay ~/ruby/coderay:possible-speedups rake bench
/Users/murphy/.rvm/rubies/ruby-2.3.0/bin/ruby bench/bench.rb ruby html 3000
encoding 3000 kB of ruby code to html...
run 1:  1.49 s, 2010 kB/s
run 2:  1.49 s, 2011 kB/s
run 3:  1.49 s, 2014 kB/s
run 4:  1.50 s, 1994 kB/s
run 5:  1.49 s, 2017 kB/s