Version 2.1.0

• New #104: Add new static methods Logger::assertLevelIsValid(), Logger::assertLevelIsString() and
  Logger::assertLevelIsSupported() (@vjik)
• New #108: Support of nested values in message templates' variables, e. g. {foo.bar} (@vjik)
• New #109, #113, #116: Add context providers (@vjik)
• New #111: Add DateTime and DateTimeImmutable support as time in log context (@vjik)
• New #112: Add Message::category() method and Message::DEFAULT_CATEGORY constant, deprecate
  CategoryFilter::DEFAULT in favor it (@vjik)
• New #113: Add Message::trace() method (@vjik)
• New #114: Add Message::time() method (@vjik)
• Chg #104: Deprecate method Logger::validateLevel() (@vjik)
• Chg #109: Deprecate Logger methods setTraceLevel() and setExcludedTracePaths() in favor of context provider
  usage (@vjik)
• Chg #116: Deprecate methods setCommonContext() and getCommonContext() in Target class (@vjik)
• Chg #118: Replace gettype() to get_debug_type() in exception messages generation (@vjik)
• Bug #84: Change the type of the $level parameter in the Message constructor to string (@dood-)
• Bug #89: Fix error on parse messages, that contains variables that cannot cast to a string (@vjik)
• Bug #98: Fix error on formatting trace, when it doesn't contain "file" and "line" (@vjik)