add option to disable regexp host matching

[123hurray]

MatchHost function treats host string as plain string but also regexp, which might cause some unexpected condition, like foo.bar.com matches foo-bar.com.
However nock can distinguish plain string and regexp (regexp in js surrounded by /)

This PR adds request.DisableRegexpHostMatching to treat host only as string. After calling DisableRegexpHostMatching, MatchHost will not using regular expression. Regexp host matching is enabled default to keep compatibility.

Before:

gock.New("http://foo.bar.com").
		Get("/bar").
		Reply(200).
		JSON(map[string]string{"foo": "bar"})
http.Get("http://foo-bar.com/bar") // match

After calling DisableRegexpHostMatching

gock.New("http://foo.bar.com").
		DisableRegexpHostMatching().
		Get("/bar").
		Reply(200).
		JSON(map[string]string{"foo": "bar"})
http.Get("http://foo-bar.com/bar") // not match

[h2non]

What about this instead:

gock.New("http://foo.bar.com").
  WithOptions(gock.Options{ DisableRegexpHost: true })

[123hurray]

What about this instead:

gock.New("http://foo.bar.com").
  WithOptions(gock.Options{ DisableRegexpHost: true })

Great! Will update it.