Configure custom email alert template
    
    Plain text with relevant details for better usability.
    Not perfect, has uneven empty lines between the sections, but
    usable.
    
    A templates directory is created which can be utilized for other
    Prometheus templating needs if more come needed.
    
    Signed-off-by: Georg Pfuetzenreuter <mail@georg-pfuetzenreuter.net>