Add g:circomp_insert_first_candidate option.
circomp#start() enhancement.

Now we can specify a completion mode to start.

circomp#start("\<C-n>")
    start with the circular completion index which key is <C-n>.

circomp#start("\<C-n>", -1)
    start with the circular completion index which key is <C-n>.
    completion mode searching is performed in reverse order.