/clock slice

[martyychang]

This pr adds support for a /clock slice command, which can be used to tag an amount of time (ultimately measured in minutes) on a time entry for reporting purposes. Sample commands below.

/clock slice 30 minutes qa
/clock slice 45 minutes build
/clock slice 2.5 hours training

A few other notable changes below.

• Reverted the matches implementation for earlier commands to the simple means of checking the starting characters in the command text
• Introduced a custom label for purposes of storing message templates