Add vendored cargo feature

[kpcyrd]

This is already the default behavior of the zstd crate, but this feature allows the following:

[feature]
vendored = ["openssl/vendored", "zstd/vendored"]

[dependencies]
openssl = "0.10"
zstd = { version = "0.13", features = ["pkg-config"] }

With a Cargo.toml like this, the default behavior would by dynamic linking for both openssl and zstd, and a static binary could be built with:

cargo build --target x86_64-unknown-linux-musl -F vendored

At the moment, the Cargo.toml would need to be written like this:

[feature]
default = ["zstd/pkg-config"]
vendored = ["openssl/vendored"]

[dependencies]
openssl = "0.10"
zstd = "0.13"

And built like this:

cargo build --target x86_64-unknown-linux-musl --no-default-features -F vendored

I also made sure that, even with the vendored feature set, one could still opt into dynamic linking with:

ZSTD_SYS_USE_PKG_CONFIG=1 cargo build

The options take precedence in this order (lowest to highest):

1. default (vendored)
2. pkg-config (dynamically linked)
3. vendored (vendored)
4. ZSTD_SYS_USE_PKG_CONFIG=1 (dynamically linked)

Thanks!