- ffmpeg 3.3 -> 3.3.2
- added VERSIONINFO

ffmpeg 3.2.1 -> 3.3
libvpx 1.6.0 -> 1.6.1

- more aggressive cache for files under CMAKE_BINARY_CACHE
  * don't re-read files MD5
  * don't repack archives
- add support for ENV{OPENCV_DOWNLOAD_PATH}
- added ID parameter as project/group identifier
- non-flat .cache directory (based on ID)
- download message prefix based on ID
- more detailed logging via ocv_download_log() macro
- force .gitignore file for .cache folder (with '*' pattern)

Versions:
- ffmpeg 3.2.1 (was 3.1.3)
- openh264 1.6.0 (was 1.5.0)
- libvpx 1.6.0 (was 1.5.0)

Scripts are updated for Linux-based (Ubuntu 14.04) mingw cross-compilation (full stack of scripts provided)
Part of these scripts may work under Windows installation of MinGW, but it is not supported.

FFMPEG update: 2.7.1
Added OpenH264 Cisco binaries support for H264 encoding: v1.4.0