Add indel signature plot

[keller-mark]

Reference: Fig 2 Part 1 (pg 31) of Alexandrov 2018 "The Repertoire of Mutational Signatures in Human Cancer" http://dx.doi.org/10.1101/322859.

• Rows are signatures (as in SBS plot) (the referenced figure would be a single row)
• Columns are variant type "groups" (1bp deletions, 1bp insertions, >1bp deletions at repeats, >1bp insertions, deletions with microhomology)
• Columns are also divided in to "sub-groups" (e.g. within deletions with microhomology, subgroups are classified by deletion length [2, 3, 4, 5+])
• X-axes are lengths or number of repeat units (within variant type group) (e.g. for 1bp deletions, homopolymer lengths are [1, 2, 3, 4, 5, 6+], but for >1bp insertions at repeats, axis is number of repeats by sub-group)
• Y-axes are counts/fractions/proportions

[keller-mark]

Note that the colors used in the reference plot can be found here https://github.com/lrgr/mutation-signature-explorer/blob/events-and-sig-types/src/colors.js