Ensure commands and flags are well formed

[gallais]

At the moment we only guarantee that there are no duplicates but we should replace the String type with one with stronger guarantees (using fromString to provide backwards compatible surface syntax).

Here are some (debatable) constraints:

• commands should start with an ascii letter
• commands & modifiers should only use ascii letters
• commands & modifiers should not contain whitespace
• modifiers should start with -- or -