Provide features to import parts of the crate

[gendx]

For now, dependents of lzma-rs import all of LZMA, LZMA2 and XZ, even if they only need LZMA for example. This could lead to unnecessary dependencies if for example SHA-256 checksums are added for the XZ layer (#32).

In the next breaking release, the code could be split with Cargo features:

• by default, lzma is available,
• the lzma2 feature enables lzma2 on top of it,
• the xz feature enables all of lzma, lzma2 and xz,
• the sha256 feature enables support for SHA-256 checksums (instead of returning an Err) - only relevant for xz.

[gendx]

• The enable_logging feature shouldn't bring the env_logger dependency - env_logger should be a dev-dependency only.

[chyyran]

This doesn't need to be a breaking release if default imports everything. It would be an opt-out rather than opt-in but can be done without a major version increment.

Might I also suggest having encode being it's own feature as well?

[gendx]

This doesn't need to be a breaking release if default imports everything. It would be an opt-out rather than opt-in but can be done without a major version increment.

If something already depends on lzma-rs with default-features = false, it would break them. So this is a breaking change. But given that lzma-rs is still not stable (currently version 0.2.0), I think that it's totally fine to increment the major version (i.e. 0.3.x).

Might I also suggest having encode being it's own feature as well?

Sounds good, especially given that the current implementation doesn't try to compress much (see #9).