Allow required fields to be highlighted instead of optional fields

[kbayliss]

Addresses #68

Adds a new TBXFORMS_HIGHLIGHT_REQUIRED_FIELDS setting which will mark required fields with * instead of marking optional fields with (optional).

Also:

• adds some missing tests
• allows developers to more easily target the required/optional <span> elements with two new classes: .tbxforms-field_marker--required and .tbxforms-field_marker--optional, allowing each variant to be independently styled.

Example when TBXFORMS_HIGHLIGHT_REQUIRED_FIELDS=False or not set (default behaviour):

Example when TBXFORMS_HIGHLIGHT_REQUIRED_FIELDS=True:

[kbayliss]

@zerolab @siimonevans Hola both! Does this address your needs?

[zerolab]

One non-blocking suggestion. This would do the trick rather nicely