org-pcomplete: Modify completion for buffer keywords
* lisp/org-pcomplete.el (pcomplete/org-mode/file-option): Collect
valid keywords for completion without requiring the whole export
framework.
(pcomplete/org-mode/file-option/options): Rewrite using new export
framework. Only complete up to the colon.
(pcomplete/org-mode/file-option/x): Removed macro.
(pcomplete/org-mode/file-option/title,
pcomplete/org-mode/file-option/author,
pcomplete/org-mode/file-option/email,
pcomplete/org-mode/file-option/date): Removed functions.
(pcomplete/org-mode/file-option/infojs_opt): New function.
Completion for some keywords was removed because it provided only the
default value, which is the same as not writing the keyword at all. In
other words, a user would almost always want to modify the completed
value, which makes the completion moot.