This allows setting some properties of the toot directly while editing
the message. It is backwards-compatible with the use of # for comment
lines and forwards-compatible with the introduction of new metadata
comments in the future. The supported ones are listed in the explanatory
message appended to the text in the editor.

In case of errors, the composed toot is fed back into the editor,
including a message about what went wrong.

The command-line arguments are reset so that if the corresponding
metacomment is removed the default value for the parameter is used
again. This ensures that the output from the editor is consistent.

As discussed on IRC, remove the filtering of lines starting
with a hash-sign. This allows pasting snippets of code that with # as
comment symbol, and allows hashtags to start on an empy line.

Metadata support is moved to a YAML-style header similar to what Jekyll
uses. We don't go full YAML though, just a simple key: value syntax
(the same as before, but without the # in front).