Additional helm actions

Hi,

I really enjoy using this package, but I still find myself using `helm-org-agenda-files-headings` sometimes. Mainly because I would like to insert a link to a heading. I also have written a few additional actions that I use with `helm-org-agenda-files-headings` which are useful for my purposes and have not yet found the time to port them to `helm-org-rifle`.

Maybe `helm-org-rifle` could consider adding some or all of the following actions:

- Insert link to heading
- Schedule heading
- Capture at heading
- Mark heading as default task (see `org-clock-default-task`)

The `Capture at heading` action would work by supplying a new option `helm-org-rifle-capture-template-key` which will be used as input to `org-capture` with `point` at the selected heading. Or instead of adding a new option, just explicitly create a new sub-heading.