Add Conversion Function Highlighting

[engineerjoe440]

I realized that there was no existing support for the standard IEC 61131-3 conversion functions (BYTE_TO_BOOL, etc) so I crafted some RegEx matches that add this support. I'll open a pull request (#13) for this, but for clarity, here's the main portion that I'm suggesting:

I would love to hear if you have any other thoughts regarding this suggestion, or if you feel that anything should be changed.

With this new portion, syntax highlighting appears for these conversion functions similar to what might be found below:

Thank you!