Modify the playerctl metadata [KEY] command to accept multiple keys

[b0o]

Example:

$ playerctl metadata xesam:artist xesam:title mpris:length xesam:userRating
Bonobo
Outlier
475000000
1

See also: #64

[acrisci]

IFS=$'\n'
metadata=($(playerctl metadata artist title))
echo "Now playing: ${metadata[0]} - ${metadata[1]}"

[acrisci]

With #73, the better way to do this is:

playerctl metadata --format 'Now playing: {{ artist }} - {{ title }}'