Allow _custommessage suffix to the bumpversion:file section header

So this becomes valid:
[bumpversion:file_myfile_first_pass:version.txt]
[bumpversion:file_myfile_second_pass:version.txt]

Fixes c4urself#111

Inspired by andrivet#3

bumpversion/cli.py

@@ -279,14 +279,14 @@ def _load_configuration(config_file, explicit_config, defaults):
 
     part_configs = {}
     files = []
-    file_or_part = re.compile("^bumpversion:(file|part):(.+)")
+    file_or_part = re.compile("^bumpversion:(file(_[^:]+)?|part):(.+)")
     for section_name in config.sections():
         section_name_match = file_or_part.match(section_name)
 
         if not section_name_match:
             continue
 
-        section_prefix, section_value = section_name_match.groups()
+        section_prefix, _file_suffix, section_value = section_name_match.groups()
 
         section_config = dict(config.items(section_name))
 
@@ -306,7 +306,7 @@ def _load_configuration(config_file, explicit_config, defaults):
                 **section_config
             )
 
-        elif section_prefix == "file":
+        elif section_prefix == "file" or section_prefix.startswith("file_"):
             filename = section_value
 
             if "serialize" in section_config: