By default, sshcode will now start a master connection with no command
so that users only need to authenticate once and multiple connections
don't need to be established. This speeds up load times significantly as
there are less handshakes required.

To disable this behaviour you can use `--no-reuse-connection`.

- Replace the `ssh -O exit` tidyup command with just a SIGTERM on the
  master
- Add `exec` to the front of the SSH master cmd so it replaces the `sh`
  process (so we can send SIGTERM to it easier)

Checks if the master process is running by sending signal 0 to it.
According to kill(2), sending a signal of 0 will send no signal but will
still perform error checking.

To prevent the SSH master from becoming a zombie process, a wait call
was added in a goroutine.

Checks:
- if it exists
- if it's a directory (if not warn and disable reuse connection feature)
- if it has safe permissions (not writable by anyone except the owner,
  if not then warn)

Update --bind flag help text

Solves proxy issue, closes coder#74 when proxy is set at .profile

Add SSH master connection feature

Replace wget with curl