Changed the syntax for specifying filters

Some examples

4grep g subject=programming     Using the full filter name
4grep g s=programming           Short version

4grep g subject=programming email!=foo@bar.com   Negates the 'email' filter

4grep g body,subject=programming   Assigns the same value to body and subject
4grep g bs=programming             Shorter version
4grep g bs!=programming            Negates both filters
removed underscores from filter names and cleared README
Removed BOOL_FILTERS (bumplimit, imagelimit, etc)
Removed and changed some specifiers in format_threads()
Renamed --output to --format
Renamed --lazy to --one
Updated README.md
s/negate/invert/
s/filter/field/
added more error handling in parse_argv()