Add GNU style long command line arguments

We are running out of letters for the command line options, which are becoming increasingly less memorable, so this is an attempt to introduce GNU style long arguments like --self-gravity to extend the range of possible options and make them more self describing.

I've added a small amount of code from another project, rather than using getopt_long, as the latter is not POSIX, and the code I found is well written and provides a good basis for what we need. The licensing is compatible with the GPL, although not the same (MIT/X11).

This also brings the documentation of the option to the same place as the definition and allows grouping, so we have simulation and basic control options separated.

@nnrw56, @matthieu, @jborrow, @lhausammann how does this look to you?

The next stage would seem to be to release some of the single character options from the less used options...

argparse/FAQs.md

+# FAQs
+
+## Why removing parsed command-line switches/options?
+
+It destroys the original `argv` array, not compatible with other arguments parsing
+library.
+
+This is because this library is used for short-lived programs, e.g. cli tools
+at beginning. It's very convenient to process remain arguments if we remove
+parsed command-line arguments,  e.g. `<comamnd> [-[s]|--switch]... arguments`.
+
+If you want keep original `argc/argv`, you can make a copy, then pass them to
+`argparse_parse`, e.g.
+
+```c
+int copy_argc          = argc;
+const char **copy_argv = argv;
+copy_argv  = malloc(copy_argc * sizeof(char *));
+for (int i = 0; i < argc; i++) {
+	copy_argv[i] = (char *)argv[i];
+}   
+argparse_parse(&argparse, copy_argc, copy_argv);
+```
+
+Issues:
+
+- https://github.com/cofyc/argparse/issues/3
+- https://github.com/cofyc/argparse/issues/9
+
+## Why using `intptr_t` to hold associated data? Why not `void *`?
+
+I choose `intptr_t` because it's a integer type which also can be used to hold
+a pointer value.  Most of the time, we only need a integer to hold
+user-provided value, see `OPT_BIT` as example. If you want to provide a pointer
+which points to a large amount of data, you can cast it to `intptr_t` and cast
+it back to original pointer in callback function.