fixed bug in seqtk's trimfq that didn't trim entire runs of low qual regions

This bug occurred because stretches of low quality (such that s in
line 217 is never above 0) never set beg or end in line 218. This
commit just checks that max has been changed from its default value 0,
and if not (and there's never a high quality base above max's default
value of zero), the beg is 0 and the end 0+max_len.
choose (2n+1)-th or (2n)-th reads
r32: convert fastq quality
Merge pull request #12 from vsbuffalo/trimfq-branch

fixed bug in seqtk's trimfq that didn't trim entire runs of low qual regiona
added tabtk
cut at both SPACE and TAB
remove duplicated columns
added tabtk examples
added comments
split the inner loop for 10% speedup
added numerical stats

Not quantile, though. Precise quantile needs a bit effort.
added (simple) quartile, stddev and skewness
allow to skip lines; also skip when conv fail
missing header