Make issue_number optional
Make issue_number optional

When no issue number is given on the command line attempt to extract it from the current branch.  That being the case use it as the issue and only check to see if a pull request is being made; for example:

```
[0][~/Projects/openslate/list-api(bugfix/2247-fix-list-api-rule-se:af28cea)]
[berto:env@theMBP]$ issue-branch --pr -u rca
```

rca/issuebranch