Add support for project-local config directories (.httpie/config.json)

[kova1max]

Closes #1229.

This PR adds discovery of a project-local config directory: a .httpie directory containing config.json, looked up from the current working directory upwards (nearest match wins) — the same discovery convention as .git or .npmrc.

Motivation

The global config applies to every invocation, which pollutes unrelated requests (see #1229). $HTTPIE_CONFIG_DIR solves this only partially — it has to be exported in every shell, which is cumbersome and doesn't work well in teams. A per-project config that is simply picked up when working inside the project directory tree solves both #1229 and, indirectly, the per-host use case of #809 without the localhost collision problem described in #1229.

Design decisions

• A directory (.httpie/) rather than a single .httpierc file, so the existing Config machinery works unchanged and sessions/plugins get project scoping for free.
• config.json must exist inside — a bare .httpie directory is ignored, so stray directories can't silently hijack configuration (and the legacy ~/.httpie behavior is preserved unchanged).
• Explicit $HTTPIE_CONFIG_DIR always wins, keeping the current escape hatch.

Resolution order: $HTTPIE_CONFIG_DIR → nearest .httpie/config.json upwards from cwd → platform defaults (unchanged).

Security consideration

Auto-loading config from a directory tree means a cloned repository can inject default_options (e.g. --proxy, --verify=no). This mirrors the long-standing behavior of npm's .npmrc. If you'd prefer, discovery could be gated behind an opt-in global setting — happy to adjust.

Checklist

• Tests added (tests/test_config.py)
• Full test suite passes (the two test_encoding.py big5 failures reproduce on clean master and are unrelated)
• Docs updated (docs/README.md)
• Changelog updated