[clang-doc] Add a structured HTML generator

Implements an HTML generator.
Nodes are used to represent each part of the HTML file. There are TagNodes that
represent every HTML tag (p, h1, div, ...) and they have children nodes, which
can be TagNodes or TextNodes (these nodes only have text).
Proper indentation is rendered within the files generated by tool.
No styling (CSS) is included.

Committed on behalf of Diego Astiazarán (diegoaat97@gmail.com)

Differential Revision: https://reviews.llvm.org/D63857

llvm-svn: 365687