0.21: Caches & Crashes

Fixed

• Improper cache invalidation for nested dependencies (#1831). Thanks @BobElDevil!
• Improper cache invalidation for prebuilt binaries (#1850). Thanks @erichoracek!
• Crash from Swift 3.1 compiler issue (#1866). Thanks @jdhealy!
• Crash from unwrapping of URLs from git (#1806). Thanks @jdhealy!
• Optional description in error output (#1884). Thanks @Nirma!

Added

• Support for custom log paths (#1863). Thanks @aaroncrespo!

Improved

• copy-frameworks now copies frameworks concurrently (#1841). Thanks @erichoracek!
• --cache-builds now uses the Swift version of the framework instead of the Xcode version (#1896). Thanks @erichoracek!

Thank you to @ikesyo, @mdiep, @scottrhoyt, and @erichoracek for improvements to the codebase! Thank you to @mdiep for reviewing pull requests.