Fix error message when vim-cursorword is installed

[brentyi]

Hi -- thanks for the great plugin!

I've been getting this error every time I install vim-cursorword:

This is because highlight groups are currently only added on the VimEnter & Colorscheme events, neither of which will be triggered unless Vim is closed & then re-opened.

Proposing to move the cursorword#highlight call to cursorword#matchadd. Adds a few more clock cycles to the timer callback, but should be insignificant.

[itchyny]

I suggest restarting the editor after installing a plugin. I think it's common to use VimEnter as the trigger of initialization and I don't like to cover this kind of edge cases. This change introduces a highlight check which always succeeds after the setup.

[brentyi]

Understood; thanks!