:heart: README cc: @amedeo @lleirborras @Thorsson
:unamused: Fix typo in README.md
:sunrise: Add Stradivari's romanticized image

Because every good gem has a captivating image in their README.md :stuck_out_tongue_winking_eye:
Update README.md
Update stradivari.gemspec
Add some docs to the README :heart:
:neckbeard: Minor documentation tweaks
:rage1: Y U NO PREVIEW
:hammer: Add a link to IFAD.org in the License
:wrench: Let's use a more fancy summary
:neckbeard: Call column title procs in the view context
:blush: de-spaghettify sortable handling @lleirborras
:beer: Allow `nil` columns
:no_mouth: Make the first column param optional
:revolving_hearts: Cosmetic
:microscope: Clean up parameter passing across generators

Rationale:

- Different generators handle their options differently. E.g.
  tabs_for doesn't have a "data" to generate tabs upon, while
  the others do

- All generators do parameter forwarding to the definition block,
  and forwarded parameters are everything after the required ones.
:neckbeard: Separate Field#content and #value, for more composability with other frameworks