feat: adopt docopt-config 0.3.0

[ralsina]

Adopts docopt-config v0.3.0 in all three CLIs (markterm, markmark, markpdf), which shipped several improvements since the v0.2.0 integration.

What we gain

• --print-config (all three CLIs): dumps the fully-resolved configuration — CLI arguments, environment variables, config file and docopt defaults merged — as YAML that can be saved and used as a config file as is:

  $ markpdf --print-config --style book
  ---
  page_size: a4
  margin: "20"
  style: book
  language: en

  markterm/markmark still require their file argument, so use e.g. markterm --print-config - < document.md.
• Environment variable type coercion: MARKTERM_HYPHENATE=1 is now a real boolean, and repeatable options accept comma-separated lists (MARKPDF_FONT=a.ttf,b.ttf).
• Better failure modes: invalid arguments print the message and usage to stderr and exit 1 (v0.2.0 printed the doc and exited 0); a config file that exists but cannot be parsed produces a warning on stderr instead of being silently ignored. Missing config files remain silent.

Adaptation required by this PR

v0.3.0 preserves config-file floats and large integers as Float64/Int64 instead of stringifying them. Cli.option_string now handles all numeric types, so margin: 20.5 keeps working instead of being dropped. Cli.option_flag shrank to value == true since the library now coerces env values itself.

Verification

• shards build: all four binaries build
• crystal spec: 231 examples, 0 failures
• ameba: 0 failures
• README help blocks verified identical to the actual --help output
• Smoke-tested: precedence (CLI > env > config > defaults), env bool coercion, env comma-split font lists, fractional margins, broken-config warning, usage-error exit codes, and the --print-config roundtrip (dump → save as config.yml → same effective configuration)