Provide CLI support for Custom Functions

[am11]

Based on #644, we need Custom Functions support for CLI, on a par with API usage.
The functions will have same behavior as importers, except for multiplicity: we can have multiple functions, but we can't have multiple importers (yet).

Options:

• Provide a single, non repeatable flag --functions, which accepts path value of type string. This path would be a path to .js file with multiple modules.exports. We will use require() and transform into arrays of functions.
• Provide a repeatable flag --function, which takes path to file with single or multiple module.exports. We will again use require().

Caveat:

(as noted here)
We need to figure out a way to make watcher aware of any changes made in custom function and custom importer, after the first require(). We would probably need to re-require all the modules (after clearing the require.chache['importer/functions']).

[am11]

Merged #815.