* aerc: add space after definitions
* aerc: only generate files, if options were set
* aerc: improve file permission warning
* aerc: remove redundant access to builtins
* aerc: allow overwriting of derived values
the order of merging the config subsets did not allow the user to specify
outgoing, source and password command values,
if they were previously derived from the SMTP, IMAP, Maildir etc config.
The values from `account.<name>.extraAccounts` now have the highest precedence.
Appropriate tests were added as well.
* aerc: write primary account first
This adds support for configuring email accounts, with automatic smtp, imap,
sendmail (msmpt) and maildir (mbsync, offlineimap) setup in aerc,
via `accounts.email`.
