hterm 1.18: Cursor shape change sequences.

* Implement cursor shape changes via DECSCUSR.
* Implement cursor shape changes via OSC 50 CursorShape (as described in
  http://vim.wikia.com/wiki/Change_cursor_shape_in_different_modes.)

BUG=None
TEST=hterm_test.html, 68/68 tests passed.

Change-Id: I89bd9d2534f800fba1113a9bd9481f2aef7b9e84
Reviewed-on: https://chromium-review.googlesource.com/64997
Reviewed-by: Robert Ginda <rginda@chromium.org>
Commit-Queue: Robert Ginda <rginda@chromium.org>
Tested-by: Robert Ginda <rginda@chromium.org>

• hterm/doc/changelog.txt[diff]
• hterm/js/hterm_scrollport.js[diff]
• hterm/js/hterm_terminal.js[diff]
• hterm/js/hterm_vt.js[diff]
• hterm/js/hterm_vt_tests.js[diff]
• nassh/doc/hack.txt[diff]