todo
Support `ask`

As an alternative to `set`, `ask` takes a key and a default value. When
the key is called with `fetch` the user will be prompted to set the
value.
Fixes to generated config file and deploy directory
Merge pull request #418 from mikespokefire/v3

Fixes to generated config file and deploy directory
allow `set` to take a proc
remove concept of deploy_user from default tasks
Focus deploy task

Placeholder descriptions added, symlink tasks now ensure parent
directories exist
todo
use pathname
clean up, prefer `roles :all` over `all_roles`
optimise mkdir commands
refactor linked file/directories dsl
add default_environment support, update templates
add uncommitted specs
todo
set defaults in a single place, call as part of setup task
typo
respect relevant configuration variables from v2
support rollback
support `primary` syntax