add prepend and append functions to seqtk seq
fix unused variable warning
add new prepend/append functions to the readme
add extra conditional to append seq if statement

I'd added it to prepend, and forgot to do it to append
fix a bug resulting from not updating seq->X.m

Also makes the realloc occur only if it needs to.
update the seq len in append too
Merge branch 'preappend'

Conflicts:
	seqtk.c