key-value pairs starting with numerics

[clach04]

Came up in #69 from @xsrvmy

Can/should keys be allowed to start with a number, i.e. 0-9? Current spec https://github.com/todotxt/todo.txt#additional-file-format-definitions implies this is currently allowed:

Developers should use the format key:value to define additional metadata (e.g. due:2010-01-02 as a due date).

Both key and value must consist of non-whitespace characters, which are not colons. Only one colon separates the key and value.

@rachmadaniHaryono recommends against using numbers which I concur with. However I'd go as far as saying the spec needs clarification here as this conflicts with how most people write times down, e.g. 11:00am doesn't seem to me to be a useful key of "11" and value of "00am".

[clach04]

Related but possibly more of an extension #64

[xsrvmy]

Another possibility is that double colon will escape a colon. This is currently simply invalid syntax since a key or a value can't contain a colon.

[jgclark]

There appears to be a universal rule (but no spec that I can find) that says #hashtags can't start with a numeral, as they often mean a list item (as indeed they do in GH).
They're not the same as keys, but I would apply the same logic.

[jgclark]

Also, borrowing from other key:value systems, all examples of YAML at yaml.org use a space character after the separator colon. I can't find this in the spec, but its common usage.
JSON too, often is written out with a space after the separator colon.
So, I suggest there has to be a good reason to disallow whitespace after the separator colon.