Add 'separate' list filter

'separate' filter allows you to convert a YAML list into a string with
each list member separated by a specified string.

Example usage:

```
---
- hosts: all
  vars:
    gpg_recipients: [ pubkey1, pubkey2, pubkey3 ]

  tasks:
    - name: Encrypt file to multiple recipients
      command: gpg --encrypt {{ gpg_recipients|separate(' -r ') }} -a /path/to/file creates=/path/to/file.asc
```