Checks if custom_wiki2html is executable on PATH

Would it be possible to add this patch.  It checks if the custom_wiki2html is an executable on the system path (rather than just a script which exists on the filesystem).

The reason I ask is that I am in the process of creating a ruby gem which handles the conversion of vimwiki markdown (github flavoured) including syntax highlighting etc to HTML.  It's all working locally so I'll be pushing it to rubygems shortly.  Anyway, I couldn't call out to the binary until I had made the attached change.

Thanks for the great work - I love vimwikiw :+1: