Fix broken demo, silent flag swallowing; add tests and LICENSE

[mtisza1]

Summary

Fixes the issues from #1:

• Broken demo (item 1). build_awk_cmd now defaults the awk program to
  {print} when neither a program nor -f is given and stdin isn't a tty,
  so something | awkplot works as a drop-in replacement for bare uplot.
  examples/demo.sh's first four steps relied on exactly this form and
  previously failed on step 1 (awkplot: awk program required...). Steps
  2 and 3 also needed -d ' ' since uplot defaults to a tab delimiter
  while awk's default OFS is a space — this was masked by the step 1
  failure and is fixed too so the demo actually runs end-to-end.
• Silent flag swallowing (item 2). Flags placed after the awk program
  (or after -f) used to be silently forwarded to awk as bogus input
  files because argparse.REMAINDER stops honoring -x-style tokens once
  the first positional is seen. awkplot now rejects any leftover
  positional that looks like a flag and isn't an existing file, with a
  hint to move flags before the program/files, instead of producing a
  plausible-looking but wrong plot.
• uplot's raw errors leaking through (item 5). Empty awk output is now
  detected before invoking uplot, printing
  awkplot: awk produced no output instead of a Ruby backtrace.
• Missing LICENSE (item 4). Added an MIT LICENSE file matching
  pyproject.toml's declared license.
• Small addition: --version flag.
• Tests (item 3, partial). Added tests/test_awkplot_cli.py covering
  build_awk_cmd, build_uplot_cmd, parse_size, and CLI-level
  --dry-run smoke tests for the two bug fixes above. pyproject.toml
  gained [tool.pytest.ini_options] so pytest runs out of the box.

CI workflow not included in this PR

Item 3 also asks for a GitHub Actions job. I could not push
.github/workflows/ci.yml with this PR because the automation's token
lacks the workflow OAuth scope (GitHub rejects pushes that add/modify
workflow files without it). Please add the following as
.github/workflows/ci.yml manually (verified locally: pytest -v and
bash examples/demo.sh both pass with this branch):

name: CI

on:
  push:
    branches: [main]
  pull_request:

jobs:
  test:
    runs-on: ubuntu-latest
    strategy:
      matrix:
        python-version: ["3.9", "3.12"]
    steps:
      - uses: actions/checkout@v4

      - name: Set up Python ${{ matrix.python-version }}
        uses: actions/setup-python@v5
        with:
          python-version: ${{ matrix.python-version }}

      - name: Install youplot
        run: sudo gem install youplot

      - name: Install test dependencies
        run: pip install pytest

      - name: Run unit tests
        run: pytest -v

      - name: Run demo smoke test
        run: bash examples/demo.sh

Not addressed

Left out of scope for this focused fix, per the issue's "smaller
follow-ups" section: --uplot-args/-- passthrough, packaging as a proper
package directory, and pinning install.sh's AWKPLOT_REF. Happy to
follow up on any of these separately.

Test plan

• pytest -v (18 tests, all passing)
• bash examples/demo.sh runs end-to-end without error
• Manual checks: bare stdin | awkplot --dry-run, flags after the awk
  program now error out, empty-input error message, --version,
  -f prog.awk, invalid -p choice still rejected by argparse

Closes #1