Crystal >= 0.34 | Release 0.20.0

[jessedoyle]

• breaking change: Remove the Duktape::Logger module and constants.
• breaking change: Remove the Duktape.logger and Duktape.headerize class methods.
• breaking change: Alert messages are no longer written to STDERR. Instead they are written to STDOUT.
• Upgrade for Crystal 0.34 support! A minimum crystal version of 0.34 is required for this release.
• Add the Duktape::Log with the Base, Alert and Console constants that act as sources for general log messages, alert messages, and console messages.
• Log messages are no longer colorized by default.
• Log output can be controlled using the newly-standardized CRYSTAL_LOG_SOURCES and CRYSTAL_LOG_LEVEL environment variables.
• Log output messages are now formatted by default as JSON with the following schema:

{
  exception : String?,
  message : String,
  severity : String,
  source : String,
  timestamp : String
}