zulip_bots: Add function to get absolute path to the bots/ dir.

This is part of our efforts to have the documentation for a
particular bot live in this repo but still be able to render it
in the main repo (in a way similar to how we render the webhooks
docs).

This function allows the calling code to get the absolute path
to the bots/ directory. This will allow us to specify an
arbitrary path (jinja2.env.loader.searchpath)
to look for templates for bots' documentation.

zulip_bots/zulip_bots/lib.py

@@ -35,6 +35,11 @@ def get_bot_logo_path(name):
 
     return None
 
+def get_bots_directory_path():
+    # type: () -> str
+    current_dir = os.path.dirname(os.path.abspath(__file__))
+    return os.path.join(current_dir, 'bots')
+
 class RateLimit(object):
     def __init__(self, message_limit, interval_limit):
         # type: (int, int) -> None