Add Dark support

[UnaNancyOwen]

Add Dark support to vcpkg_find_acquire_program().
Dark is decompiler for installer that made by WiX.
It is used to extract files from installer that made by WiX such as Kinect for Windows SDK 2.x.
How to use are as follows.

# Find Acquire Dark
vcpkg_find_acquire_program(DARK)

# Extract WiX Installer using Dark
vcpkg_execute_required_process(
    COMMAND ${DARK} -x <EXTRACT_DIR> <WIX_INSTALLER>
    WORKING_DIRECTORY ${CURRENT_BUILDTREES_DIR}
    LOGNAME extract_log
)

[ras0219-msft]

LGTM, thanks for the PR!