Support different output formats

[nuno-silva]

• Added a --format option which allows changing the output format of the program. It currently supports package.use (default), make.conf and a "plain" formats:

  $ ./cpuid2cpuflags
  CPU_FLAGS_X86: aes avx f16c mmx mmxext pclmul popcnt sse sse2 sse3 sse4_1 sse4_2 ssse3
  $ ./cpuid2cpuflags -f make
  CPU_FLAGS_X86="aes avx f16c mmx mmxext pclmul popcnt sse sse2 sse3 sse4_1 sse4_2 ssse3"
  $ ./cpuid2cpuflags -f plain
  aes avx f16c mmx mmxext pclmul popcnt sse sse2 sse3 sse4_1 sse4_2 ssse3

• Updated the README file and help message accordingly.
• Additionally, I've updated the README file with basic instructions on how to compile the program.

I'm open to feedback before merging.

[mgorny]

Thank you for your work. However, I have expressed multiple times that I'm not interested in the added complexity to support obsolete configuration formats.