Add helpers to format message text

[aaroalan]

Would be good have some helper to format text or lines.

e.g

lines = [bold('Title'), bullet('First point'), bullet('Second point'), 'End of message']
msg = FlowdockRails::Message.new(lines: lines)

# posted message should be:
#
# **Title**
# * First point
# * Second point
# End of message