Add mysql socket conn

[danvergara]

Description

This PR adds a new way to connect to MySQL through Unix Sockets. Therefore the --socket flag was added.

# URL option
$ dblab --url "mysql://user:pasword@unix(/path/to/socket/mysql.sock)/dbname?charset=utf8"

# Socket flag
$ dblab --socket /path/to/socket/mysql.sock --user user --db dbname --pass password --ssl disable --port 5432 --driver mysql --limit 50

Fixes #125

Type of change

Please delete options that are not relevant.

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] This change requires a documentation update

How Has This Been Tested?

Followed this guide to install MySQL/MariaDB on my local system.

Inspected the /etc/my.cnf.d/community-mysql-server.cnf config file to see where the socket file was.

[mysqld]
socket=/var/lib/mysql/mysql.sock

Then, I managed to connect to MySQL through that socket file as expected.

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] I have checked my code and corrected any misspellings