Made excel export format enabled by default 

Made excel export format enabled by default
Added PHPExcel as a suggested library through composer

If PHPExcel is not installed, the excel export format will output an error 
message describing that PHPExcel is not installed and direct users to read on
documentation website on how to install it.

The error message however is only displayed when someone tries to use the excel format.

Also since composer.json now suggests the phpexcel library, it is given as an option to install when installing semantic result formats.