Added glob pattern support to imapfetch.py skiplist

[jsuto]

Fixes #413

With this change, the -x / --skip-list option now supports glob patterns via fnmatch. Examples:

• -x "shared/*" — skips shared/aaa, shared/bbb/ccc, etc.
• -x "shared/*,junk,trash" — combines prefix exclusion with exact names
• -x "shared*" — skips shared, shared/aaa, sharedstuff, etc.
• -x "junk" — still works as before (exact match is a valid glob pattern)

The matching is case-insensitive to handle servers that return folder names in varying case.