Improve fzf select

Dear @lervag, I suggest three improvements for `wiki#fzf#link` which also fixes a bug. I combined them in one pull request, because they depend on each other. 1. The first commit removes `/` in the beginning of every search query in `wiki#fzf#pages` and `wiki#fzf#links`. 2. Unfortunately in the current state `wiki#fzf#link` does not work for new links, since the `mode` variable is incapsulated in existing pages. The second commit fixes this issue. 3. The last commit adds a force key for `wiki#fzf#link` similar to the one of `wiki#fzf#pages`. It is worth mentioning that I changed `g:wiki_fzf_pages_force_create_key` to `g:wiki_fzf_force_create_key` to make it usable for both mentioned selecting methods. Best regards