Implement context filtering for agenda commands and capture templates.

Implement context filtering for agenda commands and capture templates.

* org.el (org-contextualize-agenda-or-capture)
(org-rule-validate): New functions, implement context
filtering for agenda commands and capture templates.

* org-agenda.el (org-agenda-custom-commands-contexts): New
option.
(org-agenda): Use it.

* org-capture.el (org-capture-templates-contexts): New option.
(org-capture-select-template): Use it.

* org.texi (Templates in contexts): Document the new option
`org-capture-templates-contexts'.
(Storing searches): Document the new option
`org-agenda-custom-commands-contexts'.

This idea comes from Sylvain Rousseau, who implemented a similar
feature with org-context.el: https://github.com/thisirs/org-context

This implementation is a bit simpler and more general.  Simpler
because it relies on existing templates, no need to define other
contextual ones.  More general because contexts can be defined
wrt files and modes.

Thanks *very much* to Sylvain for paving the way -- certainly a
great addition to Org.