arc init support for macros

[m-butler]

Is your feature request related to a problem? Please describe.
After adding @macros followed by a macro file name in my .arc file, running arc init does not create the macros dir and associated macro js file.

Describe the solution you'd like
Create the macros dir under src as well as a js handler for the specified name (just like with other .arc declarations).

Describe alternatives you've considered
Creating the dir and js file manually.

Additional context or notes
@brianleroux asked that I enter this issue per a convo on slack.

[ryanblock]

This is a dope idea!