Provide means for overriding interactive mode logic

The builtin interactive function implemented in fasd leaves much to be desired. This PR exposes a way to override the default implementation. I use https://github.com/mooz/percol for example in this manner:

``` bash
__user_fasd_interactive_func() {
    local selector="percol --auto-match --auto-fail"
    printf %s\\n "$1" | sed 's/^[^ ]*[ ]*//' | $selector
}
_FASD_INTERACTIVE_FUNC=__user_fasd_interactive_func
```