Foliantcontrib.includes

- Allow for the starting and ending headings to be 1-character long.

- Allow to specify IDs of anchors in the `from_id` and `to_id` attributes. Support the `to_end` attribute.

- Fix include statement regex pattern. Tags joined with `|` must be in non-capturing parentheses.

- Support `escape_code` config option. Require Foliant 1.0.10 and escapecode preprocessor 1.0.0.
- Process `sethead` recursively.

- Support Foliant 1.0.9. Add processing of `!path`, `!project_path`, and `!rel_path` modifiers (i.e. YAML tags) in attribute values of pseudo-XML tags inside the included content. Replace the values that preceded by these modifiers with absolute paths resolved depending on current context.
- Allow to specify the top-level (“root”) directory of Foliant project that the included file belongs to, with optional `project_root` attribute of the `<include>` tag. This can be necessary to resolve the `!path` and the `!project_path` modifiers in the included content correctly.
- Allow to specify all necessary parameters of each include statement as attribute values of pseudo-XML tags. Keep legacy syntax for backward compatibility.
- Update README.

- Take into account the results of work of preprocessors that may be applied before includes within a single Foliant project. Rewrite the currently processed Markdown file path with the path of corresponding file that is located inside the project source directory *only* if the currently processed Markdown file is located inside the temporary working directory *and* the included file is located outside the temporary working directory. Keep all paths unchanged in all other cases.