New mappings that allow the stored `G` position to be used in both
visual mode and operator-pending mode for greater flexibility.

Previously, if `gG` (the recall mapping) was pressed before previously
having pressed a `G` command, the following error would appear:

    Error detected while processing function grappele#Recall:
    line    1:
    E121: Undefined variable: s:line
    E116: Invalid arguments for function grappele#Grappele

This happened because no previous `G` location had been saved.

Displaying an error here makes for a bad user experience. Instead if
this happens, we should just do nothing. To prevent the error, check for
the existence of the `s:line` variable before trying to use it.

Previously, we would save the location of the `G` command regardless of
whether it was used with a count or not. Thus, `34G` and `G` would both
save locations.

Saving the location of `G` isn't really useful, but it's easily
accessed. Instead, only save location when using a count with `G`. This
allows the Grappele command to be useful even after having pressed `G`.