Unify option_info lookup for normal options and view column options
Refactor view column option lookup to make it reusable
Simplify view config checking
Fix enum_name_copy method signature
Make parsing of view column expressions reusable
Add shorthand for changing the view settings of a single column

This allows to easily override the defaults from tigrc of specific view
columns without having to copy-paste the whole configuration from the
system tigrc.

Fixes #318