Added support for RST and TXT files

[dave3d]

Basically, for RST and TXT files we skip the comment parsing section, and simply pass each line of the files to the spell checker.

Also, more comments were added to the code.

[zivy]

Does this need to deal with rst markup somehow or is it sufficient to treat that as regular words and the user may need to add some that are not words into the exception/additional dictionary?

[dave3d]

Does this need to deal with rst markup somehow or is it sufficient to treat that as regular words and the user may need to add some that are not words into the exception/additional dictionary?

Yeah, it doesn't properly deal with RST markup keywords. I'm going to add those keywords to the SimpleITK dictionary.

[zivy]

Valid choice, just add this to the documentation, something along the lines of:
"reStructuredText, .rst, files are treated as standard text. Consequentially, all markup keywords that are not actual words will need to be added to the additional/exception dictionary."