Accept wildcards when specifying day of week

[Redithion]

The idea is to support wildcards when setting a day of week. A use case for this is if you want to send an alert every hour only on weekdays. Examples:

• Mon **:**
• Mon 15:**
• Mon **:15
• Mon **:**, Tue **:**, Wed **:**

This was resolved setting NOT_SPECIFIED in both min and hour when matching that case (/\A([[:alpha:]]+)\s\*{1,2}:\*{1,2}\z/).

I've also updated the README reflecting this change.

[Redithion]

I don't know why CI fails, we are using this in prod with Ruby 2.1 and Ruby 2.7