Add usage text, scdoc man page, and README

Add usage text to the program, an scdoc man page template, and a README
to the project. This is mainly so it's easier for anyone who wants to
to use secenv. It's unlikely to be used by anyone other than me, but
might as well be polite.
Rearrange argument parsing, add -T flag

Rearrange argument parsing to no longer require the use of a -f flag.
Now, any arguments following options are considered sops files until
a `--` string is encountered. So, the new command-line is:

     secenv [OPTIONS] [FILES...] -- CMD...

This is a little more convenient than using -f, since it means you can
load multiple data files in.

The template flag, -T, also allows you to pass flags on the CLI. This
isn't really recommended because the process list exposes any secrets
(the environment is also unsafe, but safer than the process list at
least). So, this is more for convenience and working with programs that
aren't friendly towards either loading encrypted data from files (ala
foo --arg @<(sops -d ...)) or environment variables.