Allow Run() on arbitrary nodes.

This is useful for dynamically running commands.

Convert env management to a resolver.

This ensures that hooks correctly detect when values are set via
environment variables. Fixes alecthomas#73.

Expand variables in config paths.

Allow binds to be provided by a function.

This is useful for situations where the initialisation of some object
should be deferred, eg. when there are distinct "setup" and "use" phases
to a tools lifecycle.

Correctly support encoding.{TextUnmarshaler,BinaryUnmarsheler}

Bind parent nodes when executing Run().

Quote enum values when displaying errors.

Validate after everything is applied.

Fixes alecthomas#27.

Hide hidden sub-commands.

A few extra fixes and tests.