Configurable macro list

[jbodah]

I'd like to see us support a configurable macro list. Macros treatment is so subjective and environment specific that just being able to setup something like g:elixir_macros = test,describe. We can specify a default list and users can add/remove values from it and the syntax rules can pick them up and treat them as necessary

[jbodah]

I'm not sure it's worth the complexity, but here's how some other folks did it https://github.com/xolox/vim-easytags/blob/master/autoload/xolox/easytags.vim#L309