This commit deactivates account selection and switching support if the
account list isn't defined. We also don't need the follow-up action of
undoing the account selection changes, so we can just skip both of the
hooks.
This is useful for users who only have 1 account - they don't need the
helm popup to select which account to use. They can just define the
variables outside of the `mu4e-account-list`.
- Clarify installation instructions
- Bring README in line with conventions
- Create layer variable for installation path
- Move mu4e-account-alist to config.el
- Move extensions.el to packages.el
- Define mu4e as built-in and not local
- Add commands to ensure mu4e is deferred
