[Feature] Add pomodoro timer feature

[replete]

Someone had a good idea the unmerged/abandoned 'daily mode' PR to add a pomodoro timer to the plugin. I think this is a good idea so this issue is for that work.

Visual comps

TODO

• Brainstorm best solution for this (manual timer? sub-task of a block of time? configurable setting?)
• Design something that makes sense
• Write UI markup
• Write styles
• Create behaviours
• Add setting to plugin options