Improve performance of SyntaxHighlightFilter

By using Rouge::Lexer.find instead of find_fancy() and memoizing the
HTML formatter we can speed up the highlighting process by between 1.7
and 1.8 times (at least when measured using synthetic benchmarks). To
measure this I used the following benchmark:

    require 'benchmark/ips'

    input = ''

    Dir['./app/controllers/**/*.rb'].each do |controller|
      input << <<-EOF
      <pre><code class="ruby">#{File.read(controller).strip}</code></pre>

      EOF
    end

    document = Nokogiri::HTML.fragment(input)
    filter = Banzai::Filter::SyntaxHighlightFilter.new(document)

    puts "Input size: #{(input.bytesize.to_f / 1024).round(2)} KB"

    Benchmark.ips do |bench|
      bench.report 'call' do
        filter.call
      end
    end

This benchmark produces 250 KB of input. Before these changes the timing
output would be as follows:

    Calculating -------------------------------------
                    call     1.000  i/100ms
    -------------------------------------------------
                    call     22.439  (±35.7%) i/s -     93.000

After these changes the output instead is as follows:

Calculating -------------------------------------
                call     1.000  i/100ms
-------------------------------------------------
                call     41.283  (±38.8%) i/s -    148.000

Note that due to the fairly high standard deviation and this being a
synthetic benchmark it's entirely possible the real-world improvements
are smaller.