allow adding '%' to volume command

[m4rch3n1ng]

now parses options such as playerctl volume 45% (which is equivalent to playerctl volume 0.45) and playerctl volume 5%+ (which is equivalent to playerctl volume 0.05+)

as a drive-by fix in [aa558c5] also avoid a heap-buffer overflow, as it was reading from index -1 when the string was empty (i.e. when calling playerctl volume ""). when running with asan, i got this error before this pr:

=================================================================
==404784==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x7b9262be110f at pc 0x563727a62061 bp 0x7ffc855a8810 sp 0x7ffc855a8800
READ of size 1 at 0x7b9262be110f thread T0
    #0 0x563727a62060 in playercmd_volume ../playerctl/playerctl-cli.c:424
    #1 0x563727a6727c in main ../playerctl/playerctl-cli.c:1267
    #2 0x7f7263f146b4  (/usr/lib/libc.so.6+0x276b4) (BuildId: 468e3585c794491a48ea75fceb9e4d6b1464fc35)
    #3 0x7f7263f14768 in __libc_start_main (/usr/lib/libc.so.6+0x27768) (BuildId: 468e3585c794491a48ea75fceb9e4d6b1464fc35)
    #4 0x563727a5f764 in _start (/home/may/projects/playerctl/build/playerctl/playerctl+0x7764) (BuildId: bfb9bf259ecef198d7b8b26cdd423b31888e79bc)