[feature] allow users use their wakatime

[Freed-Wu]

In https://github.com/irondoge/bash-wakatime/blob/master/bash-wakatime.sh#L17

    (~/.wakatime/wakatime-cli --write --plugin "bash-wakatime/$version" --entity-type app --project "$project" --entity "$entity" 2>&1 > /dev/null &)

Why not let user install wakatime-cli from their package manager (apt/yum/pacman/emerge/brew/nix)?

[Freed-Wu]

I found @alanhamlett change wakatime-cli to ~/.wakatime/wakatime-cli. Why not

bin=$(command -v wakatime-cli 2>/dev/null)
if [[ -z $bin ]]; then
  bin=~/.wakatime/wakatime-cli
fi