Increasing speed by directly encoding the regexp.

Instead of defining an array containing characters of semantic meaning
in markdown and then convert it to an regular expression used for search
and replace, we encode the regular expression directly. This increases
performance by about 20%, as the following times will show.

Both the previous and the current method was run on a 3000 character
string containing characters of semantic meaning in markdown. The string
was escaped 100000 times using #escape_markdown. Benchmarking was done
using linux's `time`.

real    0m10.228s
user    0m10.103s
sys 0m0.017

real    0m8.054s
user    0m7.983s
sys 0m0.020s