squiggle to create plots in current dir

Hi,

Right now squiggle creates plots in the same directory as the FAST5 file. When working with tarballs, these plots end up being created in .poretools_tmp, which is cleaned out at the end, with all png/pdf's erased.

This change will cause plots to be created in the current directory named basename(fast5) + '.png' or '.pdf'. We fail if there are two FAST5s in the tarball or in the arguments with the same basename, so it won't overwrite plots.

--Misha