Eml-parser

Fixes
- Fix catastrophic backtracking on url regex, add related tests for backtracking, unicode, and IPv6. (thanks malvidin)
- Add Unicode character ranges for re2. (thanks malvidin)
- Add tests for url_regex_simple, change where parens are matched in url_regex_simple, specify which re engine needs which expression. (thanks malvidin)
- Match URLs with trailing ? with url_regex_simple. (thanks malvidin)

Added
- As has been reported in 62 and 63 there can be issues with certain regular expressions (in this case URL regex) where the regex engine just runs forever (commonly referred to "catastrophic backtracking").
In order to make testing two seemingly popular (and with good cross-platform wheel support) alternative regex engines easier, two *extra* flags have been introduced:
- **regex** - for testing the [regex](https://pypi.org/project/regex/) library
- **pyre2** - for testing the [pyre2](https://pypi.org/project/pyre2/) library

**Note-1:** These are temporary extra tags which may be removed in future releases.

**Note-2:** eml_parser will transparently use regex if it is found, or pyre2 (in that order).

Changes
- *eml_parser.regex* has been renamed to *eml_parser.regexes* in order not to clash with the *regex* python module.

Changes
- Converted the documentation to mkdocs.

Fixed
- Fixed a bug in FROM header field parsing. In case the *display name* part contained an e-mail address, that one was naively used instead of properly parsing the field.

Changes
- Cleanup example scripts.

Fixed
- Handle extra case of when chardet detects VISCII text which Python is currently unable to decode (thanks cccs-rs 59).

Added
- Add multipart boundary marker as discussed in 56, in order to easier distinguish parts.

Fixed
- Fixed a major bug which resulted in not all URLs being returned because of a variable which was overwritten instead of being extended.
- Handle URL parsing issue and only emit a warning with the problematic URL but do not break the rest of the parsing.
- Filter out any scheme-only URLs.
- Make sure the URL parsing regex only matches URLs with scheme (as it is supposed to).

Changes
- Try to detect partial URLs (looking for a scheme) and extend the sliced body window accordingly. This allows for better URL extraction.

Fixed
- Prevent routing.parserouting() from throwing an exception on unparsable receive lines (thanks kinoute 54).

Changes
- Do not unnecessarily call *eml_parser.decode.robust_string2date* on an empty string.