Added --description-file=DESCRIPTION_FILE option to allow a text file
containg a lengthy review description to passed to post-review.

Added a check to confirm that --description-file and --description are not
both used as they are to be considered mutually exclusive.

Added a check to confirm that the specified file exists.

Tested the following:

post-review --help to confirm expected usage information is displayed.
post-review --description="cmd line dec" --descritpion-file="/tmp/test.txt" to confirm error generated if these mutually exclusive options are used together.
post-review --descritpion-file="/tmp/deoes_not_exist.txt" to confirm that an error is generated if the specified file doe not exist.
post-review --descritpion-file="/tmp/desc.txt" against a local server to confirm that a multi-line file with indendation appears as expected in the generated review.