Changed string concatenation to list extend/join to reduce memory allocations.

How to reproduce:

Test data:

  $ echo '{' $(for i in {1..10000}; do printf '"key%d":"value  %d", ' $i $i; done) '}' > /tmp/one-line.json

Before:

  $ time ./pygmentize -O encoding=utf-8 -O stripnl=False -f html -l 'json' > /dev/null < /tmp/one-line.json

  real	0m5.597s
  user	0m4.400s
  sys	0m1.158s

After:

  $ time ./pygmentize -O encoding=utf-8 -O stripnl=False -f html -l 'json' > /dev/null < /tmp/one-line.json

  real	0m0.347s
  user	0m0.309s
  sys	0m0.029s