Added the -i option that allows modifier keys that should be ignored to be specified. Such as xbanish -i shift -i control.

Right now, only lower case names are supported, I shall change that.

When debug is enabled, ignored keystrokes are mentioned.
Tidied the code a little and updated the usage section.
Tidied the code a little and replaced the use of tolower with strcasecomp, as advised by @jcs.
Removed the unnecessary + from the getopt argument.
Merge pull request #6 from HarveyHunt/ignore

add -i option to ignore modifier keys