allow the display of insertion/deletion base counts

[davidlougheed]

addresses #1498

Adds 4 new config options for alignment tracks:

• insertionTextColor (default: white)
• showInsertionText (default: false)
• deletionTextColor (default: black)
• showDeletionText: (default: false)

if both insertion and deletion text are shown via the above options, it may look something like this (example with CCS reads + HTT expansion; ignore the read colours):

there is also a bit of logic to decide when to stop drawing these labels when the user is zoomed out enough.

this also changes bases to be drawn on top of insertions/deletions to match the Java version. this can be reverted fairly easily if desired. gaps are now also drawn underneath insertions to display more intuitively.

If this PR is merged, I agree to transfer the copyright of these changes to the copyright holders of the igv.js repository.

[jrobinso]

Nice, thanks!