Implements #364

[Tythos]

#364 suggestion implemented with several small adjustments:

• arguments are not used as the source of replacement value definitions; they are instead evaluated against environmental variables (from the user runtime) identified by the replacement string in the action definition.

An example working degit.json can be seen here:

https://github.com/Tythos/degit-template-zigexe/blob/master/degit.json

In this example, all instances of the pattern r/{{project_name}}/ will be replaced by the value of the environmental variable $PROJECT_NAME. (This makes it easy to pass/forward values from other calls, or inline within the same shell command, without adjusting any of the argument parsing behavior. Documentation to README has also been added to reflect this usage.