Add command line options.

Unfinished.
Implement replacing files with hard-links
Fix wording in fdup.1
Add .gitignore

Also, try more eagerly to remove unwanted files when performing
make clean.
Rebroke text in license header
Reformulations to the manual text
Implement matcher module

The module match.c separates the functionality to match files from fdup.c. This
id the first step toward more modularity in fdup.
Use new matcher in fdup.c
Tweak rule for fdup.tar to include git revision
Fix duplicate CFLAGS line in Makefile
Implement options -b and -S
Update manpage
Add README

The file README only exists on the branch master