parsers: regex: check if regex from template is a string

[rmilecki]

Incorrectly formatted templates may result in passing OrderedDict as a
regex. That makes re.findall fail like:

  File "/home/foo/invoice2data/src/invoice2data/extract/parsers/regex.py", line 33, in parse
    matches = re.findall(regex, content)
  File "/usr/lib64/python3.6/re.py", line 222, in findall
    return _compile(pattern, flags).findall(string)
  File "/usr/lib64/python3.6/re.py", line 289, in _compile
    p, loc = _cache[type(pattern), pattern, flags]
TypeError: unhashable type: 'collections.OrderedDict'

Handle such case by displaying a meaningful warning.

[rmilecki]

I experienced this while trying to debug #441

[bosd]

Lgtm

[rmilecki]

This sould also help with #339