Exclude remote files from global hardhat mode

This is important because if you have a TRAMP remote file open but can't
currently access that host and you call "global-hardhat-mode", it will
try to connect to that host and hang indefinitely, since "file-truename"
on a remote file requires a connection to the remote host.
Merge pull request #6 from DarwinAwardWinner/no-remote

Exclude remote files from global hardhat mode
document TRAMP limitation
Merge pull request #8 from DarwinAwardWinner/master

Make global-hardhat-mode customizable via M-x customize