Add 'how' person got their term: election or succession.
Merge pull request #43 from GPHemsley/presidential-succession

Fix terms for presidents who were elected in their own right after they finished their predecessor's term. Ensure no discontinuity in term dates.

Add 'how' person got their term: election or succession.

Addresses issue #41.
add vice presidents to executive.yaml (merging pull request #47 from @gphemsley)
assign new GovTrack IDs to vice presidents who never served as president or in Congress
Agnew mistakenly listed as a president
Add 'how' values for vice presidents. Fix Johnson/Wilson merge conflict.