develop: Add feature to load value for an individual setting from a file

This feature allows configuration setting values to be loaded from a file.  This is useful for values that would be hard to enter on the command line, e.g. complex JSON structures.

Syntax: `omero config set --file <file> <setting>`

This also allows for bulk loading a number of setting values to be stored in a directory (here called `settings`):

    cd settings
    for setting in *
    do
        omero config set --file $setting $setting
    done