reference sequence with lowercase characters

[tkonopka]

Thanks for this useful tool. Straightforward to use.

Some fasta files contain lowercase characters (e.g. 'a' instead of 'A') and these characters cause the rendering to stop. The errors are due to lookups of color hex codes for the lowercase characters.

    dr.line([(x, y11), (x, y21)], fill=COLOR[base], width=self.xscale.base_width)
KeyError: 'a'

I looked into the code and can submit a pull request with a fix.