Implementation for --no-code-tag-removal flag

[jofas]

Fixes #153.

[ekalinin]

Thanks for the PR. A few things need to be fixed before it can be merged:

1. Conflicts with master. Since this PR, --depth (#174) and --numbered (#177) took the same argument slots: $8/$9 in gh_toc and $3/$4 in gh_toc_grab. Please rebase onto current master and move the flag to the next free slots. Watch out for local no_code_tag_removal=$3 in gh_toc_grab: it merges without a conflict, and since tags are only removed when the value is exactly "no", master's depth=0 there turns backticks on for everyone. Checking for "yes" instead would keep the default safe. Also, the blank line added in tests.bats conflicts with the "TOC for remote README.md" test, which is re-enabled on master, so please keep master's side there.
2. The flag doesn't work with stdin. It's parsed after the - branch returns, so gh-md-toc - --no-code-tag-removal silently strips the backticks, and gh-md-toc --no-code-tag-removal - treats - as a file name (it prints the file header and footer and rm: illegal option -- ~). Parsing it together with --indent/--depth/--numbered, before the - check, would fix it. Please add a stdin test too.
3. HTML entities end up inside the backticks. GitHub renders `a<b>` as <code>a&lt;b&gt;</code>, so the entry becomes [The `a<b>` case]. Entities aren't decoded inside code spans, so GitHub shows a<b> literally, while without the flag the same heading displays correctly. The entities need to be decoded when converting to backticks.
4. Code that contains backticks breaks. Every <code> becomes a single backtick, so ``foo `bar` baz`` turns into `foo `bar` baz`, which is two code spans with a bare bar between them. The fence has to be longer than the longest run of backticks inside the code.

[jofas]

The entities need to be decoded when converting to backticks.

Do you have a proposal for how to implement this? The list of entities is huge and implementing this ourselves would be unfeasible. There are options available, but I'm not sure if adding a dependency for this is something you are willing to do / if you are, what dependency to choose.