Label each nucleotide in SVG with the index of input sequence

[blakesweeney]

It would be very helpful for integration with other data sources if all nucleotides in the svg output of R2DT included a data-sequence-index (and something similar in the JSON format) that indicated the index in the input sequence of R2DT for each sequence index in the diagram. This will be very useful for mapping any sequence level information into R2DT diagrams for RNAcentral.

We cannot assume that all nucleotides in input sequence are present in a diagram. As an example where it is extremely unclear consider: https://rnacentral.org/rna/URS000042DCC3/33903. This sequence is 1,441 long but the diagram, at least according to the parsing I do for RNAcentral, is only from 9 to 1438 of the original sequence. We could modify the SVG's when importing into RNAcentral but I think a lot of useful dyanmic features will be powered by adding this data to the result of R2DT. For example, providing the ability to upload some value and color nucleotides by the data would require this.

Generalizing this to include template index may also be useful.