site.json: Give an easier way to specify which files to exclude from the build process

[yamgent]

If there's a directory that the user does not want MarkBind to traverse during the page rendering, he must modify the glob's regex to specifically exclude such directories. For example:

"**/index.md" --> "{,!(book)/**/}index.md"

to exclude index.md files residing in book directories.

I suggest that we provide a user an alternative way to ignore entire directories directly with an "ignore" parameter for "pages":

  "pages": [
    {
      "glob": "**/index.md"
    },
    {
      "ignore": "book/"
    },

[acjh]

I feel that this isn't appropriate as glob explicitly asks MarkBind to process files that match this pattern and generate pages.

If this were to be implemented, it should:

• definitely be outside the "pages" array as it doesn't specify a page to be rendered;
• maybe have "exclude" in its name rather than "ignore" to avoid ambiguity.

[ang-zeyu]

Moved to #715