Added support to multi-melody RTTTL files

fix bug which causes the crash at boot in esp8266 core v3.0.x

NOTES: The unordered_map is replaced by static array: now the initialization of the notes mapping is more efficient and the sketch size is reduced by 5kb

- add support to LittleFS

- fix extended note duration (dot notation) in rtttl format when note duration is equal to 1
- getNote(..) returns an empty note instead of crashing when accessing to invalid position

fix compile error on new arduino cores, add license

increase bpm limitation to 300 and init platformio

added selection of standby voltage level

New features:

    - Support RTTTL format
    - Simultaneous play of different melodies
    - Melodies can be "moved" among Players

Complete library rewriting, the API has completely changed, but the f…

…unctionalities aren't changed.

First version