List entries and strike 'em

[martyychang]

This pr introduces the two commands below.

• /clock entries will list all time entries from the last seven days in reverse chronological order based on Start Time. Each listed entry will have a numeric index next to it, for use with the /clock strike command.
• /clock strike will delete the time entry at the given index, as listed by /clock entries

[martyychang]

I manually tested this in Slack with the following commands.

/clock in 9:00pm
/clock out 9:30pm
/clock strike 1

/clock in 9:30am
/clock out 9:45am
/clock strike 4