Release version 0.42.0.
Announce 0.42.0 release.
Update documentation for 0.42.0.
Add a little more description of exchange.vim plugin.
Use <code/> for samples and keys.

Having these properly marked with a tag allows the CSS to highlight them.
Before a mixture of single quotes, double quotes and <tt/> was used.

Fixes #408.
Add information about @: register.
Add CSS styling for <code> blocks.
Add some more <code> tags to documentation.
Add a couple more <code> tags and tweak <code> css.
Fix whitespace.

File was a mixture of tabs and spaces, revert to spaces.
Prevent cursor movement to save selection.

When switching from insert to temporary visual mode, the insert mode
leaving commands would move the caret. This would then result in erratic
behaviour, including resetting the selection upon a double-click.

Fixes #402.
Fix caret type when UI selection is used.

Since Vrapper isn't driving the selection, the caret type might need to
be switched depending on the direction and type of selection.

This doesn't apply for exclusive selection mode where the caret is a
simple vertical bar.