Add stg email format and stg email send

These commands are wrappers for `git format-patch` and `git send-email`,
with the key feature of allowing patch names from the StGit series to be
specified on their command lines.

A subset of these commands' options, hopefully the most useful ones, are
exposed directly by `stg email` such that they are documented for StGit
users. Other options for `git format-patch` and `git send-email` may still
be passed at the end of the command line, after a '--' separator.

N.B. `git send-email`'s various '--smtp-*' options were intentially
excluded from `stg email send` with the hope that users will statically
configure SMTP via `git config`.

Compat:
- Mail aliases are now handled with `git send-email`'s mechanism, i.e. the
  `sendemail.aliasesfile` configuration variable. The old `mail.alias.*`
  configuration is no longer used.

Author: jpgrayson