Added all the necessary to do GetLanguageByAlias functionality works

internal/code-generator/generator/test_files/aliases.gold

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+package slinguist
+
+// CODE GENERATED AUTOMATICALLY WITH gopkg.in/src-d/simple-linguist.v1/internal/code-generator
+// THIS FILE SHOULD NOT BE EDITED BY HAND
+// Extracted from github/linguist commit: fe8b44ab8a225b1ffa75b983b916ea22fee5b6f7
+
+// languagesByAlias keeps alias for different languages and use the name of the languages as a alias too. All the
+// keys (alias or not) are written in lower case and the whitespaces has been replaced by underscores.
+var languagesByAlias = map[string]string{
+	"asp":               "ASP",
+	"aspx":              "ASP",
+	"aspx-vb":           "ASP",
+	"au3":               "AutoIt",
+	"autoit":            "AutoIt",
+	"autoit3":           "AutoIt",
+	"autoitscript":      "AutoIt",
+	"bat":               "Batchfile",
+	"batch":             "Batchfile",
+	"batchfile":         "Batchfile",
+	"bsdmake":           "Makefile",
+	"c++":               "C++",
+	"cpp":               "C++",
+	"django":            "HTML+Django",
+	"dosbatch":          "Batchfile",
+	"go":                "Go",
+	"golang":            "Go",
+	"gradle":            "Gradle",
+	"html+django":       "HTML+Django",
+	"html+django/jinja": "HTML+Django",
+	"html+jinja":        "HTML+Django",
+	"htmldjango":        "HTML+Django",
+	"make":              "Makefile",
+	"makefile":          "Makefile",
+	"mf":                "Makefile",
+	"njk":               "HTML+Django",
+	"nunjucks":          "HTML+Django",
+	"obj-c":             "Objective-C",
+	"objc":              "Objective-C",
+	"objective-c":       "Objective-C",
+	"objectivec":        "Objective-C",
+	"winbatch":          "Batchfile",
+}

internal/code-generator/generator/test_files/aliases.test.yml

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+---
+ASP:
+  aliases:
+  - aspx
+  - aspx-vb
+AutoIt:
+  aliases:
+  - au3
+  - AutoIt3
+  - AutoItScript
+Batchfile:
+  aliases:
+  - bat
+  - batch
+  - dosbatch
+  - winbatch
+C++:
+  aliases:
+  - cpp
+Go:
+  aliases:
+  - golang
+Gradle:
+  type: data
+HTML+Django:
+  aliases:
+  - django
+  - html+django/jinja
+  - html+jinja
+  - htmldjango
+  - njk
+  - nunjucks
+Makefile:
+  aliases:
+  - bsdmake
+  - make
+  - mf
+Objective-C:
+  aliases:
+  - obj-c
+  - objc
+  - objectivec