Libify

[Jake-Shadle]

This PR makes changes so that this crate it can be used as a library (eg rust-minidump/minidump-writer#21 (comment)).

• Replaced failure with anyhow. failure has been unmaintained for several years, anyhow is one of the maintained alternatives. This change is intended as temporary, as IMO the errors in this crate should use thiserror, or just manually defined errors, as it makes library usage easier by not using string errors for everything, but made sense for a binary application.
• Places clap and simplelog behind a cli feature flag, these dependencies are only used when used as a binary, crates that depend on this as a library don't need them and thus should not compile them. To preserve backwards compatibility for cargo install this feature is made default, so library users will need to explicitly opt-out.
• Places HTTP symbol retrieval behind the http feature. This removes several heavy dependencies only used for retrieving symbols from remote symbol stores, in the use case that motivated this PR linked above, this functionality is not needed and only adds compile time for no benefit. Again, this feature is enabled by default for backwards compatibility.
• Replaces reqwest's use of native-tls with rustls, simplifying the build when http is enabled. I frankly despise OpenSSL and all of the aggravating problems that come with it, rustls is a drop-in replacement that completely avoids all of those aggravations. This change is obviously due to my personal bias, I can back it out if requested.

Resolves: #253

[gabrielesvelto]

LGTM