- Add two new options, keyfile and certfile, which must be both set for
  the auth to work.
- if both are set, then poezio doesn’t force-prompt a password if there
  is none specified
- add /cert_add, /cert_fetch, /cert_disable, /cert_revoke and /certs
  commands.
- add a page of documentation on the process

through overloading the class variable default_config.
also fix a bug that would add meaningless sections to plugin
configurations.

fix #2841

Might fix some not-yet-seen bugs

Make the "default" parameter optional and thus move it to the end of
the command with the other optional parameters.

And change all the calls.

the Config object now also optionally takes a dict of default values
in its constructor.

- Add two new options: tmp_image_dir and extract_inline_images
- tmp_image_dir is $XDG_CACHE_HOME(usually ~/.cache)/poezio/images if unset
- Name the images from a SHA-1 of their data and their mimetype
- Output file:// links inside the message

(still surgically, without touching comments or anything else)

Will also allow a remove_in_file or whatever to remove options instead
of blanking them.

fixes whitespace issues, some builtin overrides, and some enormous lines

might make poezio run nanoseconds faster!

delayed execution is cleaner and less error-prone

(thanks eijebong)

…………………………

config.get('option', 'value').lower() == 'value' is just ugly and
stupid, especially for bool.

One if in basetabs:556 was also missing a comparison, leading to True
whenever the option was set.

(should prevent some conditions leading to config
corruption happenning when poezio cannot write anymore)

- log_errors option, true by default
- errors go in log_dir/errors.log (so $XDG_DATA_HOME/errors.log
    by default)

This should help a lot for debugging, and provide a way for people
to easily give debug traces without useless or personal infos.

(use new default directory, install the plugins as a separate
python module…)

Also fix the copy of the default config if -f is used

- Do not crash because of low disk space
- Notify the user whenever it happens
- A few functions now return a boolean instead of nothing
- Config.silent_set is Config.set_and_save without toggle and returning
  strings. It is used whenever we don’t need set_and_save
- Config.set_and_save now returns a tuple (that can be passed directly
  to core.information())

TODO: display the precise error to the user (instead of “unable to…”)

(also move replace_key_with_bound() to core.py, to prevent having
common.py depending of config.py)

fix #2184