Allow to select user when assigning an issue

[lalit2054]

Command -
jira i <issueId> -a <user>
Requirement -
It should allow to present the list of users matching user parameter as most of the time we don't know the exact user name. The list to be presented as selectable options.

[lalit2054]

I have created a Pull Request for the same. It is working perfectly fine for me. Kindly check and merge into master. PR #51