Add an optional 'Default' column

[benjaminfox]

As suggested by @jorisroling:

Many devices have 'defaults' other than 0 (for 0-based) or 64 (for centred) parameters. An example would be Elektron having Key Follow defaulting to 32 very often.

To accomplish this I will:

• add two columns to the existing definition files:
  • cc_default_value
  • nrpn_default_value
• update the CSV template that new contributions use
• update the readme