Insert tilde between a pair of regexps
(Hopefully) properly insert tilde between some characters
Ignore prefix argument if tilde is inserted
Handle expression spanning two lines
Missing tildes are reported as regions

* Function that search for missing tildes takes into accout also tab and
  newline characters.
* Fix function checking previous word by ignoring whitespace characters
  before a point.
* Fix function checking if point is between some characters by taking
  into accout text up to to first occurrence of whitespace character
  only.
* Update function adding new overlay and ert tests to reflect introduced
  changes.
Constant to use in `skip-chars-*` functions
Insert tilde once when pressing space multiple times