skip 0 index for negative start indices

Hi,
when visualizing an mRNA, often indexing is "start-codon-based", ie. first position of AUG is 1 and all preceeding indices are numbered decreasingly starting with **-1**

Currently, Forna also uses the "0" index, ie eg using

```
>rna|start=-1
AAUG
```
will result in the indexing -1, 0, 1, 2 instead of -1, 1, 2, 3

thanks for fixing,
best,
Martin
