Check whether inline parsing can be speed up by using strpbrk() about markdown

[cebe]

Played around a bit with xhprof and refactored parseInline() to use strpbrk() instead of foreaching through all markers with strpos(). Improvement makes it about 20% faster on the original markdown spec document.

38852df
bc36caf

[cebe]

$ bin/markbench benchmark
Runtime: PHP5.5.1-2
Host:    Linux cebe-laptop 3.2.0-4-amd64 #1 SMP Debian 3.2.32-1 x86_64
Profile: Basic markdown content with all official syntax / 1000 times
Class:   Markbench\Profile\DefaultProfile

+----------------------+------------+---------+---------------+---------+--------------+
| package              | version    | dialect | duration (MS) | MEM (B) | PEAK MEM (B) |
+----------------------+------------+---------+---------------+---------+--------------+
| cebe/markdown        | 1.0.x-dev  | gfm     | 3215          | 7864320 | 7864320      |
| cebe/markdown        | 1.0.x-dev  |         | 3251          | 7864320 | 7864320      |
| erusev/parsedown     | dev-master |         | 3860          | 7864320 | 7864320      |
| michelf/php-markdown | 1.4.x-dev  |         | 8176          | 8388608 | 8388608      |
| michelf/php-markdown | 1.4.x-dev  | extra   | 11125         | 8126464 | 8126464      |
| kzykhys/ciconia      | 1.0.x-dev  |         | 17013         | 8912896 | 9175040      |
| kzykhys/ciconia      | 1.0.x-dev  | gfm     | 19690         | 9175040 | 9437184      |
+----------------------+------------+---------+---------------+---------+--------------+