Update config option to accept full filepath

[mnalband]

Negative (file not found) examples to show what config filepath was used:

Without env variable:

(py2)[mnalband@mnalband work_repo]$ unset DID_DIR

Default config path:

(py2)[mnalband@mnalband work_repo]$ did
 ERROR  Unable to read the config file '/home/mnalband/.did/config'.

Only config filename:

(py2)[mnalband@mnalband work_repo]$ did --config config2
 ERROR  Unable to read the config file '/home/mnalband/.did/config2'.

Filepath and filename:

(py2)[mnalband@mnalband work_repo]$ did --config /tmp/config2
 ERROR  Unable to read the config file '/tmp/config2'.

With env variable:

(py2)[mnalband@mnalband work_repo]$ export DID_DIR=/tmp/did

Default config path:

(py2)[mnalband@mnalband work_repo]$ did
 ERROR  Unable to read the config file '/tmp/did/config'.

Only config filename:

(py2)[mnalband@mnalband work_repo]$ did --config config2
 ERROR  Unable to read the config file '/tmp/did/config2'.

Filepath and filename:

(py2)[mnalband@mnalband work_repo]$ did --config /tmp/config2
 ERROR  Unable to read the config file '/tmp/config2'.

Positive usage example with filepath and filename

(py2)[mnalband@mnalband work_repo]$ did --config /tmp/config 
Status report for the week 32 (2017-08-07 to 2017-08-13).

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Maryna Nalbandian <mnalband@redhat.com>
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

* Highlights

* Joy of the week

[psss]

Looks good. Thanks for the improvement, Maryna. And sorry for the terrible response time ;-)