Add highlighting for Igor Pro procedures

Available in pygments since 5ceb7533e214.
For a background of IGOR Pro see www.wavemetrics.com and www.igorexchange.com.

Repositories with igor procedure files:
- https://github.com/stakahama/igorlib
- https://github.com/yamad/igorutils
- https://github.com/dleifohcs/srs-igor
- https://github.com/david-hoffman/Igor-Data-Analysis
- https://github.com/bmcguyer/DAQ_Procedures_for_Igor_Pro

Pygments has a special style for igor pro. Is there a way I can force a specific pygments style? 
```sh
pygmentize -L styles | grep -A1 igor
* igor:
    Pygments version of the official colors for Igor Pro procedures.
```