Host names that start with an exclamation mark don't make much sense in the list of hosts

And since the entry is located within a `Match` directive, evaluating `Match` directives probably doesn't make sense either.

Sample configuration:
The host `ignored_host` appears in the widget's list as `!ignored_host`.

```ssh_config
Match !canonical host !ignored_host
  CanonicalizeHostname yes
  CanonicalizeMaxDots
  CanonicalDomains foo.com bar.com
```