Create global config override option

In order to take advantage of a configuration override from the command
line, it must be set globally somehow via an option. This paves the way
for that option.
Add '--config-manager' option for configuration override

In order to set a global configuration override, it must be defined
using

    Resque::Pool.config_override = ->(config) { {} } # callable object

If this is placed on it's own in a configuration file of some sort, you
can tell resque-pool to use that file with the command line option:

    resque-pool --config-manager ./path/to/override/config.rb