Initial creation of global_vars extension (#137)

Author: kogi

global_vars/README.md

@@ -0,0 +1,13 @@
+# Git Resource
+
+Author: [Bob Jackman](https://github.com/kogi)
+
+An extension for reading/writing global variable values
+
+## Usage
+
+```python
+set_global('foo', some_value)
+print(get_global('foo'))
+unset_global('foo')
+```

global_vars/Tiltfile

@@ -0,0 +1,11 @@
+def get_global(name):
+    return os.getenv(_get_env_name(name))
+
+def set_global(name, value):
+    os.putenv(_get_env_name(name), value)
+
+def unset_global(name):
+    os.unsetenv(_get_env_name(name))
+
+def _get_env_name(name):
+    return 'TILT_GLOBAL_VAR_%s' % name.upper()