Add emphmodechars opcode

[dkager]

Re: feature/ueb_update_code

For the Dutch braille tables (defined in nl-g0.utb) it is desirable to have an emphmodechars opcode. The behavior is similar to capsmodechars, with the following differences:

• The opcode works on the italic, bold and underline typeforms.
• The opcode does not affect the detection of these typeforms. Rather, it specifies which braille symbols are defined to end 'emphasis mode', thus requiring another emphasis indicator if emphasis continues beyond these symbols.

The default behavior without this opcode defined is to consider all characters to continue 'emphasis mode'. Examples (assume the entire phrase is emphasized):
Without emphmodechars:

• test-test gets one emphasis indicator before the first letter.
• test!test gets one emphasis indicator before the first letter.

With emphmodechars -:

• test-test gets one emphasis indicator before the first letter.
• test!test gets two emphasis indicators, one before the first letter and one right after the exclamation mark. This is because the exclamation mark is not defined to continue 'emphasis mode'.

Edge cases to further investigate:

• !test
• test! versus test!
• !!!

[dkager]

This has been superseded by #99. I had some trouble with the implementation, so will suspend this and instead try to generalize capsmodechars after the opcode unification/renaming is done.

[bertfrees]

See https://github.com/liblouis/liblouis/pull/747/commits

[egli]

Fixed in #747