Add a way to run commands from a file

`sqlcmd` provides a [`:r`](https://docs.microsoft.com/en-us/sql/tools/sqlcmd-utility#sqlcmd-commands) meta-command to run commands from a file in the current session. `psql` spells it [`\i`](https://www.postgresql.org/docs/current/static/app-psql.html). Does this feature exist in `mssql-cli`? I tried both `:r` and (for the heck of it) `\i` to no avail, though the latter did pop up a tantalizing file browser. 🤔

```
$ mssql-cli -d foo
Version: 0.8.0
Mail: sqlcli@microsoft.com
Home: http://github.com/dbcli/mssql-cli
foo>:r bar.sql
Msg 102, Level 15, State 1, Line 1
Incorrect syntax near ':'.
```


```
$ mssql-cli -d foo
Version: 0.8.0
Mail: sqlcli@microsoft.com
Home: http://github.com/dbcli/mssql-cli
foo>\i foo.sql
Msg 102, Level 15, State 1, Line 1
Incorrect syntax near '\'.
```