Generation of images for transcripts

This patch allows to generate images for transcripts, where the ruler coordinates start from a negative value, the A of the ATG is at the position +1 and the preceding position is -1 (and not 0).

It is done by adding two new properties to the style file: a numerical property "ruler_offset", which specify a value to add to the positions in the ruler before printing them (which can be a negative value, as required for obtaining the correct figure for transcripts) and a boolean property "ruler_nozeropos", which specifies if an additional position shall be subtracted whenever pos <= 0, so that coordinates axis skips the position zero.
I agree, in the default case it should be 0. I'd be happy to merge this one after this small change.