Better documentation for custom key bindings

Fixes #170
Bind Alt+F7 to ClearHistory

cmd/PowerShell Windows mode supports clearing history with Alt+F7, so PSReadline will too.
Fix exception after certain keywords

Rendering in PSReadline assumed all input had corresponding tokens,
but the PowerShell parser doesn't return any tokens after in at least
one case - after input like 'process foo'.

The fix is to gracefully handle running out of tokens.  Fixes #148.