Fixes for users with grep aliases

The biggest change is that I converted `grep` to `\grep`. I noticed that trying to open a bash shell hung forever b/c I had aliased grep to have some flags I use often and this broke fzf when sourcing. Strangely enough, this didn't affect zsh though. I also modified the install script to make sure that `~/.fzf.${shell}` exists before sourcing which also means the uninstall script no longer needs to remove those lines. I version control my dotfiles which are on numerous machines, some without fzf. I figured other people may do the same. Also some users may prefer `.` as opposed to `source` so I modified `append_line` to optionally just check for `~/.fzf.${shell}` instead of `source ~/.fzf.${shell}`.