Added support for simple macros

By simple regex substitution, the plugin now has support for simple
macros (no arguments). You must edit the 'macros' tiddler as follows:

   <command name> <number of arguments> <command definition>

each column must be separated by TABS (not spaces).
Added support for 1-argument macro

Now it is possible to define 1-argument macros, like \vector{v} standing
for \vec{v}.