Use verbose strings on `EtteMode` configuration, instead of integers

[arnaldopereira]

This change updates both the code and also the README. Beware that it does not add any test yet and it contains another change ahead of itzmeanjan/ette.

[itzmeanjan]

Hello, thanks for PR. I think it's better to do a regex matching than case sensitive string matching.

[arnaldopereira]

If we use a regex, shouldn't we use it on the configuration keys too? IMO the configuration could be strict enough to accept specific values (ie: not a regular expression) and have a sanity check, to validate the config and bails on bootstrap.

[itzmeanjan]

I've stopped maintaining ette, so closing this PR. Thanks.