Add api_calls to shell out to stylus' javascript api.

I needed this method to create variables for custom theming. It's essentially just a simple wrapper that shells out to commands on the stylus object. Works as a class method on Stylus, e.g:

`Stylus.api "define", "mycolor", "red"`

which uses the `define` api to create a `mycolor` global variable.

Also included a passing test.  I'd like to see this in the next ruby-stylus version in some form or another, so let me know what you think!

lib/stylus.rb

@@ -23,11 +23,12 @@
 module Stylus
   extend Runtime
   class << self
-    @@compress = false
-    @@debug    = false
-    @@paths    = []
-    @@imports  = []
-    @@plugins  = {}
+    @@compress  = false
+    @@debug     = false
+    @@paths     = []
+    @@imports   = []
+    @@api_calls = {}
+    @@plugins   = {}
 
     # Stores a list of plugins to import inside `Stylus`, with an optional hash.
     def use(*options)
@@ -47,11 +48,22 @@ def import(*paths)
     end
     alias :imports :import
 
+
+    # Stores a list of api calls to execute on every compile process.
+    def api(command, *args)
+      @@api_calls[command] = args
+    end
+
     # Retrieves all the registered plugins.
     def plugins
       @@plugins
     end
 
+    # Retrieves all the registered api calls.
+    def api_calls
+      @@api_calls
+    end
+
     # Returns the global load path `Array` for your stylesheets.
     def paths
       @@paths
@@ -103,7 +115,7 @@ def compile(source, options = {})
       end
       source  = source.read if source.respond_to?(:read)
       options = merge_options(options)
-      exec('compile', source, options, plugins, imports)
+      exec('compile', source, options, plugins, imports, api_calls)
     end
 
     # Converts back an input of plain CSS to the `Stylus` syntax. The source object can be

lib/stylus/runtime/compiler.js

@@ -1,17 +1,22 @@
 var stylus = require('stylus');
 
-function compile(str, options, plugins, imports) {
+function compile(str, options, plugins, imports, apiCalls) {
   var style = stylus(str, options);
   var output = '';
 
   for(var name in plugins) {
     var fn = require(name);
     style.use(fn(plugins[name]));
   }
+
   imports.forEach(function(path) {
     style.import(path);
   })
 
+  for(var command in apiCalls) {
+    style[command].apply(style, apiCalls[command]);
+  }
+
   style.render(function(error, css) {
     if(error) throw error;
     output = css;

spec/spec_helper.rb

@@ -21,6 +21,7 @@
     Stylus.debug = false
     Stylus.paths = []
     Stylus.plugins.clear
+    Stylus.api_calls.clear
     Stylus.imports.clear
   end
 end

spec/stylesheets/api_call.styl

@@ -0,0 +1,2 @@
+body
+  color: mycolor

spec/stylus_spec.rb

@@ -62,6 +62,17 @@
     expect(Stylus.compile(input)).to eq(output)
   end
 
+  it 'stores the api calls' do
+    Stylus.api "define", "mycolor", "red"
+    expect(Stylus).to have(1).api_calls
+  end
+
+  it 'executes the given api call' do
+    Stylus.api "define", "mycolor", "red"
+    input, output = fixture(:api_call)
+    expect(Stylus.compile(input)).to match(/color: 'red'/)
+  end
+
   it 'includes and imports "nib" automatically' do
     Stylus.nib = true
     input, output = fixture(:nib)