Don't force conceallevel and concellcursor

[Nudin]

The plugin needs conceallevel to be set to 1 or 2 to work and also sets concellcursor to 'inc' so that the markers are also visible for the line of the cursor.

Since not everyone likes this behavior this commit changes the behavior:

• If conceallevel it set to 1, don't change it
• If concellcursor is nonempty, don't change it
• If concellcursor is empty but g:indentguides_concealcursor_unaltered is set, don't change it

This should be the best compromise of "works fully out of box" and "doesn't ignore users settings"

[thaerkh]

I agree with the compromise - this is great, thank you