Replace escaped delimiters by delimiters per se

Escaped delimiters remained as is instead of being replaced by the
delimiters escaped.
Replace deprecated dvipng option '-x' by '-D'

According to dvipng manual the option '-x' is deprecated and it is
recommended to replace the option by '-D'.
Simplify regexp for search text between delimiters
Read arguments for divpng from a config file
Make delimiters configurable

Running markdown with latex plugin on existing markdown text may require
to escape many latex-plugin delimiters. Making the delimiters
configurable allows the user to reduce this effort.
Merge pull request #12 from shuber2/localmods2

localmods2: making delimiters configurable