Added variables for executable and flags

[DJManas]

Hello,

simply added 2 variables and used them: platformio-executable - default "platformio" as it was. You can change the path to platformio. I had created this, because I am using ~/.platformio/penv/bin/pio and when I add it to default PATH, it messes with my other python environments,

platformio-default-flags - default "-f -c emacs" as it was. I did not need to change it at the moment, but thinking about future.

Thanks, Regards, Petr Sourek