Customizable messages on break window

[AllanChain]

See also #3

Customizable messages can be useful. While it's impossible to have the default messages suitable for everyone, it can be set to remind us of exactly things we personally need reminders of.

Maybe we can borrow some from Stretchly (License compatible):

• https://github.com/hovancik/stretchly/blob/692415cbb654281c2c997d4a99cd4f58f23e4339/app/utils/defaultBreakIdeas.js#L7
• https://github.com/hovancik/stretchly/blob/692415cbb654281c2c997d4a99cd4f58f23e4339/app/utils/defaultMicrobreakIdeas.js

Considering this is a nice-to-have feature and being relatively easy to implement, we put it as medium priority.