#297 Support -I subslitution for directive processing

[Donald Morgan]

If the processing of directives could be modified to process directive arguments as tokens with isIgnoreToken() replacement, then the -I option could be used to cause selected #ifdef blocks of code to be ignored as if they are an #if 0.

For the following code example, -I DOC_C=0 would be used to cause ctags to ignore text in an "#ifdef DOC_C":

ctags -I DOC_C=0 test.c

test.c

#ifdef DOC_C
Code documentation that might have all kinds of wacky stuff that could confuse ctags.
#endif