(hack) Convert guides to YAML and pretty print

[lucywyman]

When we first implemented the guide subcommand, we chose to make the
file format of guides flat text and documented how users could add their
own guides to Bolt to be loaded by the bolt guide subcommand. While
useful, we haven't seen anyone use this functionality, and further it
does not seem like the file format has much impact on whether users add
their own guides or not. This converts the file format from flat text to
YAML, which supports both being read as data and formatting multiline
strings. This lets us format the bolt guide <guide> output to be more
readable and inline with other bolt output, make the guide text easily
editable (as opposed to JSON), and still lets users add their own guides
if they choose.

!feature

• Beautify bolt guide output (hack)
  This converts our existing guides from flat text to YAML so that they
  can be pretty printed.

[beechtom]

Since these now need to be parsed before printing, should the guides just be data in Bolt instead of separate files?

[lucywyman]

I don't think so - this still allows people to add their own guides if they'd like, plus having so much text as code is kind of awful, plus having these be JSON makes them more difficult to format so keeping them as YAML is preferable. I don't see any advantage to changing where they're loaded from.