Make mail attribute configurable

[creshal]

Currently, this is hardcoded in /lib/omniauth/strategies/ldap.rb to a few default values. Please add some way to make this configurable.

[pboling]

mapping is now an option you can configure for the entire LDAP schema! It was added in #107

[creshal]

I appreciate it, but the project I wanted to use it in was shelved nine years ago, so I won't be able to test this.