Allow to exclude/include source files from build process using filter

[ivankravets]

Implement new option to build environment in platformio.ini named src_filter. This filter will consist of these templates:

• +<PATH> include template
• -<PATH> exclude template

Path MAST BE related from src_dir. All filters will be applied in theirs order.

GLOB patterns are allowed.

By default, src_filter is predefined to +<*> -<.git/> -<svn/> -<examples/>

Documentation

http://docs.platformio.org/en/latest/projectconf.html#src-filter

Example

Need to include ALL(+<*>) source files, then exclude "some/hal"(-<some/hal>) folder, then include specific "some/hal/platform"(+<some/hal/platform>) folder.

[env:some_env_name]
platform = ...
board = ...
src_filter = +<*> -<some/hal> +<some/hal/platform>

CC: @smartanthill , @nobugs-hare, @valeros