Feature: Implement actions for sending oil entries to quickfix

[lpoto]

Closes #223 .

Exposes actions:

• actions.send_to_qflist: Sends files from current oil dir to quickfix and replaces old entries
• actions.add_to_qflist: Sends files from current oil dir to quickfix and keeps old entries
• actions.send_to_loclist: Sends files from current oil dir to location list and replaces old entries

• actions.add_to_loclist: Sends files from current oil dir to location list and keeps old entries

  The quickfix actions trigger QuickFixCmdPre and QuickFixCmdPost autocommands.

  The actions do not open the quickfix window, since you may create an autocmd to open it automatically on QuickFixCmdPost.

[stevearc]

Looks good, thanks!