Consider altering a column as backward incompatible

* Resolves #9
* With test
* For mysql only currently

django_migration_linter/sql_analyser.py

@@ -50,6 +50,10 @@ migration_tests = (
         'fn': lambda sql, **kw: re.search('RENAME TABLE', sql) or
                 re.search('ALTER TABLE .* RENAME TO', sql),
         'err_msg': 'RENAMING tables'
+    }, {
+        'code': 'ALTER_COLUMN',
+        'fn': lambda sql, **kw: re.search('ALTER TABLE .* MODIFY', sql),
+        'err_msg': 'ALTERING columns'
     }, {
         'code': '',
         'fn': has_default,