TODOs for copiable TeX

[thisiscam]

This is an updated summary for #22.

Currently selecting and copying messages will copy the unrendered TeX. However, here are some leftover TODOs that will make the UX better.

• Make formulas at begin or end of a message easier to copy.
• More robustly support copying escaped formulas like $ $10 $. This has to do with the fact that MathJax's processEscape option is ran before the copyable TeX is injected.
• Make a selected formula highlighted. This might be doable with some JS using events; or it might be possible to add a layer below the formula that is actually selectable text.

Any help or suggestion on how to fix these items are greatly appreciated!