Feat/issue 38

[barrettk]

As a user, I want to pass in a prefix to be used at the beginning of generated story IDs and test IDs

• parse_github_issues() - change where StoryId is set here
  • [x] add prefix argument (should be a string)
  • [x] if is.null(prefix) then set to repo (this will be the legacy behavior)
  • [x] zero-pad the issue number (to one more than max(as.integer(release_issues$issue)))
  • [x] put an "S" in after the hyphen and before the number (so it will be like PREFIX-S001)
• assign_test_ids() - change how Test ID is built here
  • [x] add prefix argument (should be a string)
  • [x] if is.null(prefix) then set to repo (this will be similar to the legacy behavior)
  • [x] replace middle part of ID with TEST (so it will be like <prefix>-TEST-001)

closes #38

[barrettk]

Ok now should be good lol