Form submit button plain

[olafdietsche]

Allow <button type="submit"> controls to submit forms

Forms - HTML5 and Forms - HTML4 both allow button controls as form submitter.

This patch extends _get_clickable() to allow submit buttons too.
The change from el.name and el.value to el.get(...) is needed,
because lxml parses buttons as HtmlElement and not InputElement.

[codecov-io]

Current coverage is 82.24%

Merging #1469 into master will not affect coverage as of 94e1b27

@@            master   #1469   diff @@
======================================
  Files          158     158       
  Stmts         8225    8225       
  Branches      1140    1140       
  Methods          0       0       
======================================
  Hit           6765    6765       
  Partial        264     264       
  Missed        1196    1196       

Review entire Coverage Diff as of 94e1b27

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