amidi: add sysex-interval option

amidi: add sysex-interval option

This patch adds a new option to amidi tool: sysex-interval.

It adds a delay (in milliseconds) in between each SysEx message - it searches
for a 0xF7 byte.

This is very useful when sending firmware updates to a remote device via SysEx
or any other use that requires this delay in between SysEx messages.

`amidi' manual was updated with an example usage as well.

Signed-off-by: Felipe F. Tonello <eu@felipetonello.com>
Acked-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>