Add option --load-only language codes

The user will not have to install language packages from "argos-translate" that he/she has no use for.

app/init.py

@@ -5,11 +5,11 @@ import os, glob, shutil, zipfile
 from app.language import languages
 import polyglot
 
-def boot():
-    check_and_install_models()
+def boot(load_only=None):
+    check_and_install_models(load_only_lang_codes=load_only)
     check_and_install_transliteration()
 
-def check_and_install_models(force=False):
+def check_and_install_models(force=False, load_only_lang_codes=None):
     if len(package.get_installed_packages()) < 2 or force:
         # Update package definitions from remote
         print("Updating language models")
@@ -19,6 +19,25 @@ def check_and_install_models(force=False):
         available_packages = package.load_available_packages()
         print("Found %s models" % len(available_packages))
 
+        if load_only_lang_codes is not None:
+            # load_only_lang_codes: List[str] (codes)
+            # Ensure the user does not use any unavailable language code.
+            unavailable_lang_codes = set(load_only_lang_codes)
+            for pack in available_packages:
+                unavailable_lang_codes -= {pack.from_code, pack.to_code}
+            if unavailable_lang_codes:
+                raise ValueError('Unavailable language codes: %s.' % ','.join(sorted(unavailable_lang_codes)))
+            # Keep only the packages that have both from_code and to_code in our list.
+            available_packages = [
+                pack
+                for pack in available_packages
+                if pack.from_code in load_only_lang_codes
+                and pack.to_code in load_only_lang_codes
+            ]
+            if not available_packages:
+                raise ValueError('no available package')
+            print("Keep %s models" % len(available_packages))
+
         # Download and install all available packages
         for available_package in available_packages:
             print("Downloading %s (%s) ..." % (available_package, available_package.package_version))