Make "backtraces" feature opt-in

[lucab]

This moves the "backtraces" feature out of the default set. It also
links the Backtrace implementation to the feature, so that the
public type always exists but it does not carry any backtrace content
unless the consumer explicitely enabled that.

Closes: #113

[lucab]

Rebased to latest master (and fixed a bunch of conflicts), CI is green.

[shepmaster]

My plan is to take this PR and PR #173 and ensure that they play well together locally before I merge either, then merge them separately.

Since our goal is to support the standard-library backtrace and that will be always available in some future Rust version, that's the general setup I want to mimic. I think this PR sets us up in that direction, but I don't know if it will be the final goal.

Let's assume that standard-library backtraces stabilize in Rust 1.50. My plan is that this library would grow a rust-1.50 feature flag which enables them, re-exporting it as snafu::Backtrace. Without that feature flag, Backtrace would be the inert version here. An alternate (and conflicting!) feature flag would use backtrace::Backtrace (potentially merging the backtrace-crate feature in at that point too).

[shepmaster]

I'll take care of addressing these conflicts, as part of that local testing!

[shepmaster]

Thanks!