Support adding custom commands at compile time

[zuiderkwast]

Extend gencommands.py with more flexible command handling:

• Don't require the JSON filename to match the command name.
• Don't require command name to be in uppercase in the JSON file.
• Allow non-alphanumeric chars in command name.
• Accept multiple JSON files on the command line.
• For convenience, accept files on the format of cmddef.h as input.
• Handle JSON files without key specs, with a fallback to using the "arguments"
  key to find the position of the first key.
• Handle JSON files where subcommands are defined as a simple command with a
  space in the command name (instead of as a nested JSON structure).

Additionally, correct uppercase ordering of commands where the ordering would
be ambiguous if strncasecmp() would have been used (e.g. EVAL_RO vs EVALSHA).
This fixes possible failures to find such commands in the lookup table resulting
in "unknown command" errors.

Fixes #162.

[bjosv]

Fantastic.
alloca was for me a forgotten gem, but seems sweet in this case.

[zuiderkwast]

@selcukaltinay FYI