Externalize output formatter to a dedicated function.

The dedicated function 'format_output(command, output)' can be
redefined by router and can be used to format the output according to
what is inside. In this way, the output can be formatted differently
according to the router type and the command.
Log format can be configured.

The format for each log can be configured with the configuration
option defined by $config['logs']['format']. %D is for
the time, %R is for the requester IP address, %H is for the host on
which the command has been executed and %C is for the command.

Warning! The configuration option to choose the logs file has been
moved from $config['misc']['logs'] to $config['logs']['file'].
Add documentation section in README.md.
Start documentation for the configuration options.