Read --index and --words as decimal numbers (closes #75)

Both flags were uint32 flags, which the flag library reads with Go's
base-0 rules, so `-n 010` derived key 8 and `0x10` or `1_0` were
accepted. Both now hold an options.Decimal, a flag value whose Set
parses with strconv.ParseUint(text, 10, 32): a leading zero stays
decimal, and a base prefix, an underscore, a sign or an empty value
is refused with an error naming the flag while the flags are parsed,
before any command runs. Its type is still named uint32, so --help is
unchanged and GetUint32 still reads it back. The README says the index
is a decimal number.

Model: opus-5-5

README.md

@@ -100,7 +100,9 @@ m/83696968'/<app>'/<n>'
   to one tool. Each key type's number is spelled the way BIP-85 spells its own
   RSA application (`828365` is the ASCII codes of `R`, `S`, `A` written out):
   SSH is `838372` (`S` `S` `H`), age is `657169` (`A` `G` `E`).
-- `n` is the key index: flag `--index` / `-n`, default `0`.
+- `n` is the key index, a decimal number: flag `--index` / `-n`, default `0`. A
+  leading zero is still decimal (`-n 010` is index 10); a base prefix, an
+  underscore or a sign (`0x10`, `1_0`, `+1`) is refused.
 
 ## Giving it the mnemonic