Provide option for simpler parsing of displayed equations

[matze-dd]

The current parsing of displayed equations assumes a certain style of writing formulas that may be inappropriate. On the other hand, at least inclusion of a place holder like 'V-V-V' together with trailing punctuation extracted from the formula does help the proofreading program. (Even if this omits text parts included, e.g., with \text.)

We could provide options for yalafi and yalafi.shell that switch to this simpler mode for all equation environments, which have been registered with EquEnv() in the fully detailed mode. The replacement like 'V-V-V' should be taken from a rotating collection.

See the discussion in Issue #83.