Implement more issue filters (#400)

This adds new filters to `tea issues ls` and `tea pr ls`, made available in SDK 0.15:

```
--state value                 Filter by state (all|open|closed) (default: open)
--keyword value, -k value     Filter by search string
--labels value, -L value      Comma-separated list of labels to match issues against.
--milestones value, -m value  Comma-separated list of milestones to match issues against.
--author value, -A value
--assignee value, -a value
--mentions value, -M value
--from value, -F value        Filter by activity after this date
--until value, -u value       Filter by activity before this date
```

Note: I felt free to change parameter names as exposed by SDK & API, as the names exposed by them are partially bollocks (eg `mentioned_by`) and or inconsistent with usage in other commands (eg `tea times --until`)

fixes #376, related #323

Co-authored-by: Norwin <git@nroo.de>
Reviewed-on: https://gitea.com/gitea/tea/pulls/400
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Reviewed-by: 6543 <6543@obermui.de>
Co-authored-by: Norwin <noerw@noreply.gitea.io>
Co-committed-by: Norwin <noerw@noreply.gitea.io>

cmd/pulls/list.go

@@ -24,7 +24,7 @@ var CmdPullsList = cli.Command{
 	Usage:       "List pull requests of the repository",
 	Description: `List pull requests of the repository`,
 	Action:      RunPullsList,
-	Flags:       append([]cli.Flag{pullFieldsFlag}, flags.IssuePRFlags...),
+	Flags:       append([]cli.Flag{pullFieldsFlag}, flags.PRListingFlags...),
 }
 
 // RunPullsList return list of pulls