Support wakatime-cli custom path

[enricodeleo]

As per #14 this contribution introduces support for custom-located wakatime binary.

This addresses an issue with recent homebrew's wakatime formulas that put the binary under /usr/local/bin/wakatime-cli (or /opt/homebrew/bin/wakatime-cli on M1 machines).

Usage example:

# add to ~/.zshrc
# Track usage of terminal in Wakatime
export ZSH_WAKATIME_BIN=$(brew --prefix)/bin/wakatime-cli
antigen bundle /Users/enricodeleo/Development/forks/wakatime-zsh-plugin --no-local-clone

[sobolevn]

Thanks!

[sobolevn]

Awesome, thank you!