[Feature] Add Greek (EL) locale support

[tassosgeo]

What problem does this feature solve?

ECharts lacks support for the Greek language, and it could be good to have it so that the greek users have access to the toolbox interface in their native language.

What does the proposed API look like?

var chart = echarts.init(document.getElementById('chart'), null, {
          locale: 'EL'
});