use correct string slice index
Split g:signify_sign_overwrite into two different options

The option was really describing two different pieces of functionality:

1. whether or not to remove all signs before adding signify's
2. whether to not place a signify sign on a line that has another sign

I did not want to turn on the first option, but did want the second one.

The new option is `g:signify_sign_remove_all` to go along with
`g:signify_sign_overwrite`, corresponding to (1) and (2) above
respectively.

I made it so `g:signify_sign_remove_all` defaults to
`g:signify_sign_overwrite` so that this won't break any existing
configurations out there.  Would it be better to have three total
options?  Two new ones that default to the old one? It might be less
confusing that way...