Allow simpler passing for individual host classes

Most used of Broker are for simple (single host type per operation) cases.
Becuase of this, most users passing in custom host classes are passing in a single class.
In these cases, it makes more sense to allow for an easier way to pass along the class than to specify it via key/Class mappings.

README.md

@@ -240,7 +240,7 @@ from broker.hosts import Host
 class MyHost(Host):
    ...
 
-with Broker(..., host_classes={'host': MyHost}) as my_host:
+with Broker(..., host_class=MyHost) as my_host:
     ...
 ```
 

broker/broker.py

@@ -105,7 +105,10 @@ def __init__(self, **kwargs):
             nick = kwargs.pop("nick")
             kwargs = helpers.merge_dicts(kwargs, helpers.resolve_nick(nick))
             logger.debug(f"kwargs after nick resolution {kwargs=}")
-        if "host_classes" in kwargs:
+        # Allow users to more simply pass a host class instead of a dict
+        if "host_class" in kwargs:
+            self.host_classes["host"] = kwargs.pop("host_class")
+        elif "host_classes" in kwargs:
             self.host_classes.update(kwargs.pop("host_classes"))
         # determine the provider actions based on kwarg parameters
         self._provider_actions = {}