Insert attachment 1.8

Insert attachment v1.8 changes:

Now works with XF 2.2 and newer.

Insert attachment v1.7 changes:

Fixed issue where empty variable caused server error.

Insert attachment v1.6 changes:

Now works with XF v2.0 and v2.1.

Insert attachment v1.5 changes:

Fixed typo in variable.

Insert attachment v1.4 changes:

Fixed issue dealing with inserting additional attachments if multiple attach codes are on the same line.

Insert attachment v1.2 changes:

Updated thread.php file.

Insert attachment v1.1 changes:

Updated addon.json file.