feat: Accept verbatim host matches alongside patterns

The documentation mostly makes it seem like verbatim host matches are
accepted while under the hood, gitlinker actually uses string.match().
This works most of the time, but breaks, for example, when a hostname
contains a `-`.

Both verbtim matches and pattern matches are useful features - support
them both properly by first attempting a verbatim match and then a
pattern match.

lua/gitlinker/hosts.lua

@@ -165,13 +165,14 @@ end
 
 --- Gets a matching callback for a given host
 --
--- @param target_host the host to get the matching callback from
+-- @param target_host the host to get the matching callback from.
+-- this can be either a verbatim match or a lua string.match pattern.
 --
 -- @returns the host's callback
 function M.get_matching_callback(target_host)
   local matching_callback
   for host, callback in pairs(M.callbacks) do
-    if target_host:match(host) then
+    if target_host == host or target_host:match(host) then
       matching_callback = callback
       break
     end