preload missing default props

[RaphaelDDL]

Without this default addition, README's "Audio attributes as props" table is wrong since having no props default will be the same as auto instead.

Name	Type	Default	Observable	Description
preload	String	none	true	The way to load music, can be 'none' 'metadata' or 'auto'

That means, without preload="none" in the <aplayer />, it's up for the browser to decide. In Chrome v75, all assets are downloaded, for example.

aplayer without preload props

aplayer with preload props explicitly set to none

Observe that only one was downloaded, which is the first icon, that I clicked "Play" (and is currently playing)

P.S.: I wasn't sure which branch to merge, develop or master, so sent to develop one