Support for LEB128 format as primitive type
Hi,

First of, thanks for `kaitai-struct`, it's a fantastic tool.

I am writing a parser for a format that uses [LEB128](https://en.wikipedia.org/wiki/LEB128) format (both signed and unsigned). Since the encoding/decoding of the values requires [a little bit of logic](https://en.wikipedia.org/wiki/LEB128#C-like_pseudo-code) maybe it would be cleaner to implement it as a primitive type in kaitai. I tried to implement it hackishly using value instances, but that didn't work.

I believe that would be a great extension (there is simply no tool out there that supports it), as it would allow to parse stuff like DWARF, WebAssembly and more.

Thanks.