Lf: Add a command to select all files

[alexherbo2]

[gokcehan]

@alexherbo2 You can combine :unselect and :invert commands for this purpose:

cmd select-all :unselect; invert

[mohkale]

sorry, I should've made a new issue for my previous comment, see #371.

[alexherbo2]

Don’t you think select-all merits a dedicated command?

[gokcehan]

@alexherbo2 It's not obvious what should happen to selections in other directories with a builtin select-all command. invert command should cover most use cases. In other cases where a user selects some files and then decides to select all files instead, I think it is fair to expect the user to be explicit about what should happen to the previously selected files.