Add code coverage badge

[jacalata]

Turns out github does not have a neat way to display code coverage.

I added this repo to coveralls.io. This PR adds the code coverage action to start uploading to there when it is in main, and the badge specification that will display the status from coveralls.io once that is populated.

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
tabcmd
__main__.py	10	10	10	0%
tabcmd.py	14	14	14	0%
version.py	6	3	3	50%
tabcmd/commands
commands.py	10	10	10	0%
constants.py	77	15	15	81%
server.py	130	20	20	85%
tabcmd/commands/auth
session.py	360	42	42	88%
tabcmd/commands/datasources_and_workbooks
datasources_and_workbooks_command.py	140	21	21	85%
datasources_workbooks_views_url_parser.py	140	10	10	93%
delete_command.py	60	16	16	73%
export_command.py	113	25	25	78%
get_url_command.py	127	47	47	63%
publish_command.py	87	24	24	72%
runschedule_command.py	21	7	7	67%
tabcmd/commands/extracts
create_extracts_command.py	42	8	8	81%
decrypt_extracts_command.py	27	2	2	93%
delete_extracts_command.py	37	6	6	84%
encrypt_extracts_command.py	27	2	2	93%
extracts.py	20	2	2	90%
reencrypt_extracts_command.py	27	2	2	93%
refresh_extracts_command.py	49	13	13	73%
tabcmd/commands/group
create_group_command.py	29	5	5	83%
delete_group_command.py	27	2	2	93%
tabcmd/commands/project
create_project_command.py	46	8	8	83%
delete_project_command.py	35	4	4	89%
publish_samples_command.py	30	4	4	87%
tabcmd/commands/site
create_site_command.py	34	5	5	85%
delete_site_command.py	28	2	2	93%
edit_site_command.py	38	2	2	95%
list_command.py	44	10	10	77%
list_sites_command.py	29	2	2	93%
tabcmd/commands/user
create_site_users.py	57	10	10	82%
create_users_command.py	60	10	10	83%
delete_site_users_command.py	43	5	5	88%
user_data.py	221	31	31	86%
tabcmd/execution
_version.py	2	2	2	0%
global_options.py	146	23	23	84%
localize.py	69	11	11	84%
logger_config.py	42	6	6	86%
tabcmd_controller.py	42	7	7	83%
TOTAL	2724	448	84%