WIP add functionality to drop features from GFF based on attribute values

[pjbriggs]

PR which adds a new experimental utility gff_remove_features, which removes feature records from a GFF file if specified attributes for those features match any of the supplied patterns.

(This PR tries to address the functionality requested in issue #38.)

For example:

gff_remove_features -r Parent=1002009 test.gff

will remove any features where the Parent attribute is 1002009.

Multiple patterns can be specified, and can include wildcards, for example:

gff_remove_features -r Parent=1002* -r Parent=1003* ID=GeneX test.gff

A "pattern" can also be a file, which should contain one pattern per line. For example:

gff_remove_features -r Parent=remove_parents.txt test.gff

remove_parents.txt should be a file and might look like:

$ cat remove_parents.txt
1002*
1003*