Use type alias to identify view settings and simplify option parsing
Support all option name separators when parsing view settings shorthands
Cleanup view settings parsing

This removes the lookup table by assuming the option name contains
"-view-".
Rename lineno member in argv_env to goto_line
Improve diff context display in the stage view

Reuse the diff view's select() method in both views so that file and
help information is automatically displayed. This also adds a test
of some of the different stage view features.
Fix updating of all staged or unstaged changes in a maximized stage view
Fix parsing of diff chunk header when the added lines number is missing
Add %(lineno) state variable

This exposes the current file line number as the variable `%(lineno)`.
The blame, blob, diff, grep and stage views updates the variable.

Fixes #304