default log formatters handling

Allow users to formally reference and set default log formatters
through an api on the Capistrano::Logger singleton.

- use `Capistrano::Logger.default_formatters` to refer to the current defaults
- use `Capistrano::Logger.default_formatters=` to set custom defaults
Merge pull request #324 from kellyredding/kr-default-formatters

Default log formatters handling