Add inline JSON input for the render CLI

[servletcloud]

Problem

fstache render currently accepts JSON render data only through --data PATH:

fstache render --data data.json < page.mustache > page.html

That works for files, but it is awkward for quick one-off renders, smoke tests, docs examples, and shell scripts that already have a small JSON object available inline.

Users have to create a temporary file or use shell-specific process substitution instead of writing the data directly in the command.

Proposed CLI

Add an inline JSON option, for example:

fstache render --json {"name":"Ada"} < page.mustache

--data PATH should remain the file-based option.

Design questions

• Should --json and --data be mutually exclusive?
• Should --json - mean read JSON from stdin, or should stdin stay reserved for the template?
• Should invalid inline JSON reuse the same diagnostic style as invalid --data files?

Expected behavior

• Inline JSON is parsed as the render data object.
• Invalid inline JSON exits non-zero without a traceback.
• The existing --data PATH behavior remains unchanged.
• The README CLI section documents the new option with a short example.