Writing files

[g-pavlov]

What this PR does / why we need it:
Currently, writing files and folders doesn't quite work as expected.

• Duplicate writes for files and folders
• Concurrent map writes occur
• Nodes that do not refer to documents are not handled
  among others.

What we want is a more reliable, manifest-driven materialization of the structure. And as far as hugo is concerned, smarter handling of _index.md files.

Special notes for your reviewer:

Release note:

Documents structures support "folder"-type nodes that have no contentSelector specified and result in a file folder named by the name of the node.

- Node names need not end with .md suffix to have the file written as<name>.md. If there is no .md suffix, it will be appended to the file name prior to writing it down.

Hugo support  
The following features are available when the `--hugo=true` option flag is enabled:  
- Frontmatter that should be written to a document is specified in the properties map of the corresponding documentation structure node with key "frontmatter"
- Documents frontmatter in yaml (only) is supported
- Documents with frontmatter only (no other content) and without line ending on the last line are supported.
- Documents with frontmatter preceded by any amount of whitespace are supported. Whitespace before or after the `---` start/end marks is also supported.
- Documents that start with `---` (potentially preceded by any whitespace) are considered to specify frontmatter. An error is thrown if there is no closing `---` and the content in between is not valid yaml
- Frontmatter specified as node property and frontmatter available in the node document that will be written are merged. Duplicates are not reconciled.
- A minimal frontmatter `title: <node-name-title-case>` is written when no other has been specified as node property or in the document)
- Container nodes that specify a "frontmatter" structure in their `properties` will be written as folders with autogenerated `_index.md` file that contains the specified frontmatter.
- Document nodes that need to be written as hugo section files are identified by an `index": true` entry in their Properties. Such nodes are written as _index.md files regardless of their name.
- A section file node can be inferred automatically too. A document structure node that has content selectors, and with no other peer nodes named `_index` or `_index_md`, will be written as `_index.md` file, provided that its name matches one of the entries specified in the `--hugo-section-files` flag or its default list: readme, read.me, index (case insensitive). To disable this automation, set the `--hugo-section-files` flag value to empty string. 
- Multiple files that will potentially be written as `_index.md` in the same folder are detected and reported as error.