Implement support for custom PR-table

|Q            |A   |
|---          |--- |
|Bug Fix?     |no  |
|New Feature? |yes |
|BC Breaks?   |no  |
|Deprecations?|no  |
|Tests Pass?  |no  |
|Fixed Tickets|#119|
|License      |MIT |

Allow PR-table to be repo oriented as discussed in #119, only "issue" is that using `--template=custom` is required to get the custom template to work.

The RepoCustomTemplate is Application-aware because of
late loading of the Configuration, in the future we can always improve this, but for now it works.

Small detail, I used this feature to open this PR :smile:                     

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Better name: PullRequestCustomTemplate.