also look for archetypes in env.mojito.path, if exists

Prepend "$mojito/archetypes" to the array of directories in which to
find archetype templates.

The value of $mojito is either <cwd>/node_modules/mojito if it exists,
or specified by the --libmojito option.

See also issue yahoo/mojito#1301

Note that you can use also use the full path to any directory for archetype templates.