Allow easier customization of NVBENCH_MAIN.

[alliepiper]

Moves as much of the guts of main into actual functions as possible.

The macros are now easier to reuse and redefine for users who want to customize main. Existing macro usage should continue to work.

[alliepiper]

Need to add tests for the customization points I see people using on github

[alliepiper]

Might pursue later, but this would break existing usecases, eg cudf currently does:

#include <nvbench/main.cuh>

#undef NVBENCH_MAIN_PARSE
#define NVBENCH_MAIN_PARSE(argc, argv)                                                             \
  nvbench::option_parser parser;                                                                   \
  std::vector<std::string> m_args;                                                                 \
  for (int i = 0; i < argc; ++i)                                                                   \
  {                                                                                                \
    std::string arg = argv[i];                                                                     \
    if (arg == "foo")                                                                              \
    {                                                                                              \
      i += 2;                                                                                      \
    }                                                                                              \
    else if (arg == "bar")                                                                         \
    {                                                                                              \
      i += 2;                                                                                      \
    }                                                                                              \
    else                                                                                           \
    {                                                                                              \
      m_args.push_back(arg);                                                                       \
    }                                                                                              \
  }                                                                                                \
  parser.parse(m_args)

NVBENCH_MAIN

Since NVBENCH_MAIN now just calls a function, the custom PARSE(...) macro is no longer used.