Support for 'escaped values'

[michaelklishin]

Problem Definition

Currently Cuttlefish does not support string
values that include the # character.

This character is, however, used every so often
in generated passwords, identifiers, messaging protocol
(AMQP 0-9-1, MQTTv3.1, MQTTv5) routing patterns,
and other (often machine-produced) values.

The Proposed Solution

This PR introduces an alternative value
representation:

a.setting = 'sdkjf#hsdf$82836867#9237498'

which allows such values to be escaped using
single quotes.

Given the following schema file:

{mapping, "escaped.value", "escaped.value", [
  {datatype, [string]}
]}.

{mapping, "non_escaped.value", "non_escaped.value", [
  {datatype, [string]}
]}.

and the following .conf file:

escaped.value = '2f9sd79238-7_49%#sod7f9s87ee4'
non_escaped.value = 2f9sd79238749%#sod7f9s87ee4

Cuttlefish will now produce the following app config file:

[
  %% this value was cut off at the comment character, '#'
  {non_escaped,[{value, "2f9sd79238749%"}]},
  %% '#' was preserved in this value which used single quote escaping
  {escaped,[{value,"2f9sd79238-7_49%#sod7f9s87ee4"}]}
].

Limitations and Caveats

Single quotes are NOT
supported in these escaped values by design,
I do not thing that the \' escaping would
be worth our time. They are not particularly
common in Cuttlefish schemas according to Team RabbitMQ's
extensive experience with various configuration
files in the wild.

Closes #37.

References #31.

[lukebakken]

I can merge once CI passes. Thanks!