Allow 'repl :connect' to read args from file

This basically works like in curl, you specify @filename and then the
contents of the file are literally used as if written on the CLI.

@TODO this will not work:
lein repl :connect @./some/file
because there is no realpath resolution
but this works:
lein repl :connect @/full/path
and this as well:
lein repl :connect @some/file
Merge pull request #1651 from winks/issue-1518

Allow 'repl :connect' to read args from file