Fix: Set spellcheck highlighting on every spellcheck enabling

[KSR-Yasuda]

Care for #63.

Unknown why, sometimes spellcheck highlight SpelunkerSpellBad and SpelunkerComplexOrCompoundWord can be cleared.

So, changed to set the highlightings on every spellcheck enabling.

Also, consider highlight cleared case (e.g. :highlight clear SpelunkerSpellBad called by someone).
The original code checks whether :highlight SpelunkerSpellBad output something, though,
if highlight is cleared, it outputs SpelunkerSpellBad xxx cleared.

With this fix, even if something goes bad,
you'll be able to reset by toggling spellcheck disabled -> enabled.

[KSR-Yasuda]

Updated to store highlight style.
Now it will keep customized spellcheck highlight style.

[cincodenada]

I ran into this, and this PR seems to have fixed it up - I'm running this branch in my Vim plugin for now. Thanks!