Add visual mode support for mkdx#gf

[SidOfc]

This feature request originates from #116, it would allow mkdx#gf to also function in visual mode.

Adding support for this has the following benefits:

• non-link texts that refer to files / URL's can be selected visually and acted upon.
• when the normal mode mapping fails, a visual selection can be made instead as a fallback.

This would only be enabled when g:mkdx#settings.gf_on_steroids is set to 1.

[SidOfc]

Feature has been added, both gf and gx may now be used in visual mode as well.
I decided to enable them by default instead of only when g:mkdx#settings.gf_on_steroids is enabled to keep consistent normal mode mappings function.