change chord separator

[truj]

The default value for chord separator and pattern index separator is ,.
This is not good because it prevents to pass a chord as a variable to a function or pattern call.
Another reason: It makes it harder for the parser to distinguish between a chord and a sound effect pipeline (#28).

So the chord and index pattern separators should be changed either to ; or /.
; would look pretty similar while / would be more familiar to alda programmers.

This is a breaking change, and it includes:

• updating the tutorial on the website
• updating the readme
• updating the examples
• updating a lot of unit tests
• re-testing the examples in the tutorial
• re-testing the examples in the readme
• jumping to a new version (1.6.0) - because of the breaking change

It should be done in the same branch as #28 - because #28 is so cool that it deserves to be published with a new version number.

[truj]

It doesn't make any sense to use a different character for chord separator and pattern index separator.
They should be united.