[backend] Add Rocket.Chat backend

This commit adds support to fetch
messages from a Rocket.Chat channel.

The tests have been added accordingly.
The usage docs have been updated.
Release notes have also been added.

Signed-off-by: Animesh Kumar <animuz111@gmail.com>

README.md

@@ -36,6 +36,7 @@ are:
     phabricator      Fetch tasks from a Phabricator site
     pipermail        Fetch messages from a Pipermail archiver
     redmine          Fetch issues from a Redmine server
+    rocketchat       Fetch messages from a Rocket.Chat channel   
     rss              Fetch entries from a RSS feed server
     slack            Fetch messages from a Slack channel
     stackexchange    Fetch questions from StackExchange sites
@@ -321,6 +322,13 @@ $ perceval pipermail 'http://mail-archives.apache.org/mod_mbox/httpd-dev/'
 $ perceval redmine 'https://www.redmine.org/' --from-date '2016-01-01' -t abcdefghijk
 ```
 
+### Rocket.Chat
+
+Rocket.Chat backend needs an API token and a User Id to authenticate to the server.
+```
+$ perceval rocketchat -t 'abchdefghij' --from-date '2020-05-02' https://open.rocket.chat general 1234abcd
+```
+
 ### RSS
 ```
 $ perceval rss 'https://blog.bitergia.com/feed/'

bin/perceval

@@ -73,6 +73,7 @@ are:
     phabricator      Fetch tasks from a Phabricator site
     pipermail        Fetch messages from a Pipermail archiver
     redmine          Fetch issues from a Redmine server
+    rocketchat       Fetch messages from a Rocket.Chat channel
     rss              Fetch entries from a RSS feed server
     slack            Fetch messages from a Slack channel
     stackexchange    Fetch questions from StackExchange sites