add HeaderMatchingFilter
refactor HeaderMatchingFilter to use class variables

.. for header and pattern. This makes it it easier to
define subclasses by overwriting the default values.
respect Filter._tag_blacklist

in HeaderMatchingFilter
use value of class variable Filter.tags as fallback

for 'tags' parameter of Filter.__init__
refactor SpamFilter

to subclass HeadersMatchingFilter
This is backwards compatible with the original version
in that it respects a 'spam_tag' parameter as alternative
to the 'tags' parameter inherited from Filter.
refactor ListMailsFilter

to subclass HeaderMatchingFilter
add 'lists' to default tags inf ListMailsFilter
pass on tags parameter in SpamFilter.__init__

as mentioned in
https://github.com/pazz/afew/commit/8aa05b95cf6e3c94149c63d48a28ca2b8974d7d8#commitcomment-2939393
add missing + in ListMailsFilter

we forgot to appent the plus to indicate that the 'lists' tag should
be added. Otherwise I see the following error:

```
...
  File "/home/pazz/.local/lib/python2.7/site-packages/afew/Filter.py", line 55, in __init__
      raise ValueError('Each tag must be preceded by either + or -')
```
fix: pass on tags as list of str, not a single str
expand the documentation
Move the move_mode docs into the full docs

and reformat as rst
improve command line docs page
Added utf8 encode to logger.

when running: afew --move-mails -n

I was getting the error:
Traceback (most recent call last):
  File "/home/chrisfl/.local/bin/afew", line 189, in <module>
    main(options, query_string)
  File "/home/chrisfl/.local/lib/python2.7/site-packages/afew/main.py", line 73, in main
    mover.move(maildir, rules)
  File "/home/chrisfl/.local/lib/python2.7/site-packages/afew/MailMover.py", line 62, in move
    self.__log_move_action(message, maildir, rules[query], self.dry_run)
  File "/home/chrisfl/.local/lib/python2.7/site-packages/afew/MailMover.py", line 103, in __log_move_action
    logging.log(level, "    {}".format(get_message_summary(message)))
UnicodeEncodeError: 'ascii' codec can't encode character u'\xa3' in position 58: ordinal not in range(128)

This fixes it.
add quick start page
Add more info to the installation docs
Add docs about extending afew
Removed break which meant that only a single file was getting moved on each run
Merge pull request #48 from foobacca/moredocs2

expand the documentation
Getting MailMover working on python3