fix configName lookup for user-specified config files

if the user specifies a configuration file when invoking a cli,
liftoff should use that filename as the only file to search for.

closes gh-22

index.js

@@ -55,11 +55,19 @@ Liftoff.prototype.buildEnvironment = function (opts) {
     searchPaths.unshift(cwd);
   }
 
-  // calculate the regex to use for finding the config file
-  var configNameSearch = buildConfigName({
-    configName: this.configName,
-    extensions: Object.keys(this.extensions)
-  });
+  // a list of possible configuration names
+  var configNameSearch;
+
+  if (opts.configName) {
+    // if a configName has been provided by the user, only look for that
+    configNameSearch = [opts.configName];
+  } else {
+    // otherwise, look for the default configName using all available extensions
+    configNameSearch = buildConfigName({
+      configName: this.configName,
+      extensions: Object.keys(this.extensions)
+    });
+  }
 
   // calculate configPath
   var configPath = findConfig({

test/index.js

@@ -90,7 +90,6 @@ describe('Liftoff', function () {
       expect(app.buildEnvironment().cwd).to.equal(path.resolve('test/fixtures/search_path'));
     });
 
-
     it('should resolve symlinks if config is one', function () {
       var env = app.buildEnvironment({
         cwd: 'test/fixtures/symlink'
@@ -103,7 +102,14 @@ describe('Liftoff', function () {
         configPath: 'test/fixtures/symlink/mochafile.js'
       });
       expect(env.cwd).to.equal(path.resolve('test/fixtures/symlink'));
-    })
+    });
+
+    it('should be able to find custom configuration file names', function () {
+      var env = app.buildEnvironment({
+        configName: 'myconfig.js'
+      });
+      expect(env.configNameSearch).to.deep.equal(['myconfig.js']);
+    });
 
   });