Replace md5 with hash in cached_download function for any hash algorithm

[wkentaro]

• Will support md5, sha1, sha256, ...

Usage:

# Old
gdown.cached_download(
    url=url,
    md5="xxxx...xxxx",
)

# New
gdown.cached_download(
    url=url,
    hash="md5:xxxx....xxxx",
)
gdown.cached_download(
    url=url,
    hash="sha1:xxxx....xxxx",
)
gdown.cached_download(
    url=url,
    hash="sha256:xxxx....xxxx",
)