Add support of _approximate completer

[Aloxaf]

Fix #56

[Aloxaf]

I don't know why the result of _approximate can't be inserted. This may take a long time, I will merge some useful changes to master.

[intelfx]

I just tried to merge this on top of master. After resolving two obvious conflicts, this almost works, except tht if the _approximate completer is invoked, the captured prefix inserted into fzf contains a spurious (#a<N>) prefix (apologies for screenshot, not sure how to capture fzf UI reasonably):

I guess this is related to the games _approximate plays with compadd itself:

                if (( ! $+functions[compadd] ))
                then
                        dounfunction=1
                        compadd () {
                                local ppre="$argv[(I)-p]"
                                [[ ${argv[(I)-[a-zA-Z]#U[a-zA-Z]#]} -eq 0 && "${#:-$PREFIX$SUFFIX}" -le _comp_correct ]] && return
                                if [[ "$PREFIX" = \~* && ( ppre -eq 0 || "$argv[ppre+1]" != \~* ) ]]
                                then
                                        PREFIX="~(#a${_comp_correct})${PREFIX[2,-1]}"
                                else
                                        PREFIX="(#a${_comp_correct})$PREFIX"
                                fi
                                (( $_correct_group && ${${argv[1,(r)-(|-)]}[(I)-*[JV]]} )) && _correct_expl[_correct_group]=${argv[1,(r)-(-|)][(R)-*[JV]]}
                                builtin compadd "$_correct_expl[@]" "$@"
                        }
                fi

Any ideas or pointers to start chasing?

[intelfx]

I added a dirtiest hack in intelfx@7f1d458 and _approximate seems to work reasonably with fzf-tab now. The only thing left is to maybe preselect the [corrections] group if it exists — @Aloxaf, any ideas on what's happening here or how can I do that?