add .gitignore
fasample.c: expand usage message.
fasample.c: use -f instead of -h, commonly used for help
fasample.c: add support for paired reads
fasample.c: Get rid of the -p parameter and use -f for a fraction of input.

Just printing the first X sequences is pointless. Just use head(1).

The -p option lacked usefullness. Its replaced with the new -f option,
where each input file has the specified probability of appearing in the
output. This can be done in a single pass.
add README