Add simple cli to de/compress files

[CaselIT]

Hi,

I've given a go at creating a simple cli to allow compressing and decompressing files with this library.

For the moment I've not added an setup entry point, so to run it requires requires python -m zstandard. It may be an useful thing to have though, but I think it should not be called zstd to avoid shadowing the official one

Fixes #18

[CaselIT]

Oh I noticed that this project still support python 3.5. Any particular reason to still support it, since it's eol?

I can remove the f-strings that make the test fail under 3.5 if continued support for it is needed.

[CaselIT]

Hi @indygreg
Let me know if you are interested in merging something like this or if I should instead release it as an external package.