Assign issue to contributor

[you06]

Description

Support assign issue to contributor by a command.

Option field:

assign-issue = true
// how many days will the assign lasts, 0 is unlimited
assign-duration = 7
req-change = "status/req-change"

Features:

• /assign assign issue to the user.
• /assign cancel remove assign.
• In the assign duration, other contributors can not assign this issue to them, bot will comment with "This issue is pick up by xxx(, the assign will continue until 2019-01-09 11:45:14 or submit a pull request)".
• After submitting a PR, the assign duration should be reset, if the req-change label lasts over assign duration, the issue assign will be removed.
• If the PR got merged, issue will be closed.