Add a "focus" option for the processlist plugin

[nicolargo]

This new option will allow a user to identify a list of "important" processes.

(Only those processes) or (this processes will be highlighted, display before other) will be displayed in the UI.

Configuration:

[processlist]
...
focus=<comma separated list of Glances filter>

Examples:

focus=user:foo,pid:1234,name:.*myprocess.*

[nicolargo]

Done in the develop branch.

To be tested.