Read the text form with blank lines in front, as age does (closes #77)

age decrypt took the text form only when the file began exactly with
-----BEGIN AGE ENCRYPTED FILE-----, so a blank line in front made it
read the file as the binary form and refuse it. It now looks for that
line past up to 1024 bytes of whitespace, as the age tool does, and
the text form reader skips the blank lines. A test puts a blank line
and a line of two spaces in front of a text-form file. The README's
age decrypt section says so.

Judgement call: that first line with spaces in front of it on the same
line is still refused, as age refuses it.

Model: opus-5-5

README.md

@@ -282,8 +282,8 @@ or a device, such as `/dev/null`, is written to directly.
 Decrypts the file (or standard input) with the derived identity. Output goes to
 `-o`, which is treated as for `encrypt`, or standard output. If the identity is
 not one of the recipients, the tool says so and exits with status 1. The text
-form needs no flag, and up to 1024 bytes of blank lines in front of it are
-skipped, as `age` skips them.
+form needs no flag, and blank lines in front of it are skipped, as `age` skips
+them.
 
 ## Derived mnemonics: `keyfunc mnemonic`