Parse the flags before loading the configuration to fix the error where
using the --help flag caused the app to return an error if the
configuration file does not exist.
Updated the executor lookup table by changing the map type from
map[string]Executor to map[string]func(string, bool, []string) error in
order to reduce the cost of initialising the map.
Previously the map initialised all the executors despite only needing
to run the single executor called by the command. The map now instead
maps the command to the function of type
'func(string, bool, []string) error' that will initialise and run the
executor called by the command. As a result of the change, the
Executor interface is no longer needed and has been removed.
PR: #57
