PlatformIO Library Manager manifest file

* This library in Web: http://platformio.ikravets.com/#!/lib/show/Arduino-Plotly
* Specification: [PlatformIO Library Manager](http://docs.platformio.ikravets.com/en/latest/librarymanager/index.html)
* Integration: [IDE Integration](http://docs.platformio.ikravets.com/en/latest/ide.html)

`library.json` is a library manifest file which can be used with different tools/managers to organise embedded libraries. Currently, only [PlatformIO](http://platformio.ikravets.com) uses it.  How does this look from CLI:

```bash
$ platformio lib search "arduino plotly"
# Found ... libraries:
# Arduino-Plotly        Arduino library for real-time logging and streaming data to online plotly graphs

$ platformio lib install "Arduino-Plotly"
# Installing Arduino-Plotly  library:
# Downloading  [####################################]  100%
# Unpacking  [####################################]  100%
# The library ‘Arduino-Plotly' has been successfully installed!

$ platformio lib show "Arduino-Plotly"
# Arduino-Plotly
# -------------
# Author: Plotly
# Keywords: logging, data, streaming
# Version: ***
#
# Arduino library for real-time logging and streaming data to online plotly graphs

$ platformio lib update
# Updating Arduino-Plotly  library:
# Versions: Current=***, Latest=***       [Up-to-date]
```