liblangutil: refactors SourceReferenceFormatter error formatting for pretty and colored output.

* Refactors output format in a way it is (or should at least be) more readable.
  (NB.: As source of inspiration, I chose the rustc compiler output.)
* Adds color support to the stream output.
* Also improves multiline source formatting
  (i.e. truncating too long lines, like done with single lines already)
* solc: adds flags --color (force terminal colors) and --no-color (disable autodetection)
* solc: adds --new-reporter to give output in *new* formatting (colored or not)
* Changelog adapted accordingly.

solc/CommandLineInterface.h

@@ -109,6 +109,8 @@ private:
 	std::unique_ptr<dev::solidity::CompilerStack> m_compiler;
 	/// EVM version to use
 	EVMVersion m_evmVersion;
+	/// Whether or not to colorize diagnostics output.
+	bool m_coloredOutput = true;
 };
 
 }