Support user-supplied mods and colours for multiple DNA/RNA

[sathish-t]

Dear IGV team,

Thanks for this nice repo. I am exploring ways of showing two types of DNA modifications on the view. My understanding (after exploring with Claude code) is that a mod that is not hard coded in the code is shown as a teal colour, and multiple mods are all given the same teal colour. I want to allow the user to set their own colours. I am preparing a pull request with Claude code but wanted your opinion first on this change as stated in CONTRIBUTING.md.

Here's how a user would configure it:

igv.createBrowser(div, {
  tracks: [{
    type: "alignment",
    url: "my.bam",
    colorBy: "basemod",          // or "basemod2"
    modificationColors: {
      "m": "rgb(255, 128, 0)",   // override 5mC (normally red)
      "h": "rgb(0, 200, 100)",   // override 5hmC (normally magenta)
      "27552": "rgb(1, 1, 1)"    // set color for a modification by ChEBI id
    }
  }]
})

A few notes:

• You only need to specify the modifications you want to override — anything not in the map falls through to the built-in colors
• Known modifications like m and h can be overridden, not just unknown ones

What do you think? The PR suggested by Claude is fairly short but I haven't tested if it works. I am attaching the diff file here. Thanks!

igv_PR_diff.txt

[jrobinso]

Looks o.k. After testing that it works feel free to submit a PR.