Allow to specify environment prefix when initialise project

[ivankravets]

Request is made by @ajford:

The ability to specify the env name given when running platformio init -b foo would be nice.

I have some overlapping boards (that confuse the auto-uploader, i.e. diecimilaatmega328 and sparkfun_redboard). Thus using -e is required to prevent the attempt to upload using the wrong env. But autogen_BOARDNAME is long and sometimes confusing to remember (diecimila is not easy to spell right the first time). I manually edited my platformio.ini to change my env names to easier names (i.e. freeduino, redboard, and pro16), but it would be useful if there was a way to do this when initing a project (or adding new boards, ala #167).

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Implement --env-prefix option to platformio init command.