Pre-existing bugs found while reviewing #151 #179

Found while reviewing #151. All of these reproduce on current master (9e0a858), #151 doesn't introduce them. Line numbers refer to gh-md-toc at 9e0a858.

• Remote URL ignores the HTTP status (gh_toc_load, L38). curl -s only fails on network errors, so ./gh-md-toc https://github.com/ekalinin/github-markdown-toc/blob/master/NOPE-404.md prints an empty TOC and exits 0.
• stdin input is changed before it is sent (L423-425). while read -r input; do echo "$input" >> ... strips leading whitespace, drops a last line without a trailing newline, and echo swallows lines like -n. For printf '# A\n\n # indented code\n-n\n# Last' | ./gh-md-toc - the API gets only # A, an empty line and # indented code, so the indented code block becomes a heading and # Last is lost. cat > "$gh_tmp_md" would keep the input as is.
• The stdin temp file is never removed (L421). Every gh-md-toc - run leaves a tmp.XXXXXX file with the input markdown in $TMPDIR.
• Errors go to stdout (L160-171). The network and rate-limit errors are printed with plain echo, so ./gh-md-toc README.md > toc.md writes the error text into toc.md under the "Table of Contents" header. The exit code is 1, but the error ends up in the output file.
• The rate-limit hint always points to token.txt (L168-169). It's printed even when GH_TOC_TOKEN is set, and in that case token.txt is ignored.
• Single-line HTML gives an empty TOC on macOS (gh_toc_grab, L290). BSD sed's N on the last line quits without printing, so sed -e ':a' -e 'N' -e '$!ba' ... outputs nothing for one-line input. A document with only one heading renders to one line of HTML, so printf '# A\n' | ./gh-md-toc - prints an empty TOC on macOS, while printf '# A\n\n## B\n' works.

#151 changes the stdin block and the error output, so it's easier to fix these after it's merged.