Fixed #20 - Build error on clang 12 due to outdated dependency - Updated magic_enum dependency to v0.7.3

Closed #19 - structopt now supports providing custom help message when constructing structopt::app

• Updated CMakeLists to only run MSVC related checks if the compiler is actually MSVC
• Fixed #13 - At least one positional argument is required to properly raise an error for unknown optional arguments.
• Fixed #14 - Excess positional arguments do not generate an error