vim: Fix the cursor position to run each test

* to emphasize that each selection is linewise, and
* to ensure that each test is reproducible.
vim: Add tests on vspec blocks
vim: Group tests on a plain Vim script
vim: Extract s:select_a_of for repeating
vim: Support vspec blocks
Update the document about vspec support
Merge branch 'support-vim-vspec'

Close gh-3.
Add release notes for 0.3.0
Rewrite README for clarity
Bump up version to 0.3.0
Merge branch 'prepare-for-0.3.0'