Sanitize MXP tags

[wrparker]

#27 introduced automated negotiation for TTYPE and MXP. Now we need to deal with MXP output.

MXP tags are initiated by the following characters:
<, >, and &.

For clients using MXP they must be replaced:
< = <
> = >
" = "
& = &

Input should be santized when MXP is enabled such that MXP is only sendable by the mud server and not via players in communication.

MXP should be allowed for:

• Exits
• Objects
• who list

There may be ways to abstract out player name such that anytime a player name is called, if MXp is enabled we create a linkable "whois" command.

For example:
say <send>hi</send> right now in mudlet allows linking of a "hi" command. This doesn't work in MUSHclient, but either way tags should be properly sanitized.

More information is available on this guide: https://www.gammon.com.au/mushclient/addingservermxp.htm

[wrparker]

It's also likely we need to implement some kind of secure mode for MXP. http://www.gammon.com.au/forum/bbshowpost.php?bbsubject_id=222