Add "smart" one-digit method.

Idea: Large jumps (e.g. 67 lines) are rare, so why not make most numbers
default to one-digit? So instead of using H to jump 6 lines, just use h.
To jump fifteen lines, still use ag. Numbers other than 1 default to
one-digit.

Added configuration option: Vertigo_onedigit_method can be "forcetwo",
(default) which corresponds to past behavior. "smart1" implements the
idea above. "smart2" modifies the idea so that 2 also acts like 1.
Rearrange initialization code

Move Ex commands to top, add section comments, add whitespace line
between Vertigo_homerow and Vertigo_homerow_onedigit
Enable CTRL-C for cancelling
Actually enable CTRL-C for cancelling
Add one-digit method 'smartN' for N > 2

Previously, 'smart1' and 'smart2' were hardcoded. Made general.
Functionality is the same for those options, but now supports 'smart3'
and so on.
Refactor numbering check into UsingAbsoluteNumber()
Force two digits when using absolute numbering
Improve documentation, including smart onedigit method
Merge branch 'feature/alt-onedigit'