added support for text transforms of tag names visible with '%g' in the index and pager formats

New configuration option added that is used to map a tag name to something different when displayed with '%g' in the index and pager formats. This idea was taken from the alot MUA and reduces clutter on the index line. For example I use the following:

```
tag-transforms         \                                                                                                           
    "inbox"      "∙"   \                                                                                                           
    "unread"     "⚀ "  \                                                                                                           
    "flagged"    "★ "  \                                                                                                           
    "replied"    "↻ "  \                                                                                                           
    "sent"       "➥ "  \                                                                                                           
    "signed"     "✔ "  \                                                                                                           
    "attachment" "✛ "  \                                                                                                           
    "invites"    "CAL" \                                                                                                           
    "clearquest" "CQ"  \                                                                                                           
    "perforce"   "P4"  \                                                                                                           
    "archive"    "A"   \                                                                                                           
    "deleted"    "D"
```