Use passwords from the accounts file

This commit makes it so that if the password in an account in the
accounts file is present, then use it. Otherwise ask for the password to
the user.
Connect to an account on start-up

This commit enables connecting to an account on startup which enables
the use case of having different pre-configured scripts which start
different profanity processes for different accounts. This only makes
sense in the light of not supporting many accounts connected per
process.
Merge remote-tracking branch 'tsenart/passwords-on-accounts-file'
Merge remote-tracking branch 'tsenart/connect-on-startup'
Added MAX_PASSWORD_SIZE set to 64

fixes #245
Use correct type in account rename for priorities

fixes #250
Allow "account" setting in [connection] to autconnect the account

closes #251