Parameterize mingw dockerfile

[nyurik]

This PR introduces a few improvements to the docker-based Windows builds

• compile any published branch/tag, not just the latest master branch.
  Use docker build --build-arg GIT_TAG=<tag> ...
• compile from another github repository, not just the official aria2/aria2.
  Use docker build --build-arg GITHUB_REPO=nyurik/aria2 ...
• compile local code instead of downloading the github version.
  Use docker build --build-arg SOURCE=local ...
• Control libaria2 build
  Use docker build --build-arg LIBARIA2=enable ...
• Control static build.
  Use docker build --build-arg ARIA2_STATIC=no ...
• Control HOST parameter. Uses i686-w64-mingw32 by default, but can be set to x86_64-w64-mingw32 to build a 64-bit binary.
  Use docker build --build-arg HOST=x86_64-w64-mingw32 ...
• Automatically copy aria2.exe to the shared volume /output if it is provided.
  This is significantly simpler than copying from a running container.
  Running docker run --rm -v $PWD:/output aria2-mingw will place aria2.exe into $PWD/bin directory. Additionally, if LIBARIA2=enable, libs will be copied to $PWD/lib, and aria2.h will be saved to $PWD/include/aria2/aria2.h.

This PR can be tested directly from bash without cloning:

export GITHUB_REPO='nyurik/aria2'
export GIT_TAG='win'
curl https://raw.githubusercontent.com/${GITHUB_REPO}/${GIT_TAG}/Dockerfile.mingw \
  | docker build -t aria2-mingw --build-arg GITHUB_REPO --build-arg GIT_TAG --build-arg LIBARIA2=enable --build-arg HOST=x86_64-w64-mingw32 -

# Copy results to the ./output
docker run --rm -v $PWD/output:/output aria2-mingw