Fall through to next html pattern early if tag startRe matched failed.

[soasme]

Given the usecase out2.txt provided in #48, the performance improvement is 2x.
The test result is running on my local (mbp 2.8 GHz Quad-Core Intel Core i7, 16 GB).

Before:

$ nim c -d:release src/markdown.nim && time ./src/markdown < /tmp/out2.txt
real	0m2.616s
user	0m1.897s
sys	0m0.018s

After:

$ nim c -d:release src/markdown.nim && time ./src/markdown < /tmp/out2.txt
real	0m1.751s
user	0m0.946s
sys	0m0.013s