allow option for TU to not be marked as mismatch

[vtlim]

highlight these in a different color from blue?

• right now T and U is considered a mismatch.
• right now a non-standard base letter (like H) is ignored if matching.

[vtlim]

• For edge case 1: allow user to check a box, default unchecked, for "Treat T and U as matching nucleobases?"

• Add new function in app.py for def match_TU(text1, text2)

• Call that function around lines 60-62, and update the mismatch index list to remove indexes for TU

• For edge case 2: allow placeholder non-bases (anything that is not ATGCU), e.g., for known overhanging nucleosides. No change needed.