Add build for Ubuntu 18.04 in package repository

It seems that currently the stance is that Ubuntu 18.04 is not officially supported. However, mssql-cli does seem to work for Ubuntu 18.04 and over a year has passed since 18.04 was released.

After taking a looking at this issue: https://github.com/dbcli/mssql-cli/issues/204, I ran `sudo apt-get install libicu60`.

I then installed mssql-cli using `pip`. Only issue I ran into is that I initially tried installing it using `pip3`, but then it would fail to run because it was still looking for modules with Python 2. My bad.

Anyways, I'm successfully able to run mssql-cli now. I think it's time for a build of Ubuntu 18.04 to be added to the apt package repository, and installation instructions should be provided in the documentation as well.

On a side note, there should be an example provided of connecting to a SQL Server with a custom port. Have mostly been using Postgres the past few years, didn't expect to have to specify that with `mssql-cli -S "tcp:server_ip,port"` (the `tcp:` prefix and using a comma instead of a colon were unexpected).