Support markdown syntax with HTML output

[FlorianWolters]

I think it would be great if protoc-gen-doc would support basic Markdown syntax for the creation of a HTML documentation.

Example:

Consider the following Protobuf message:

/**
 * The `DoubleDimension2d` message represents a dimension with **two** elements
 * (*width* and *height*) in double precision.
 */
message DoubleDimension2d {
  /**
   * The width of this `DoubleDimension2d` in double precision.
   */
  double width = 1;

  /**
   * The height of this `DoubleDimension2d` in double precision.
   */
  double height = 2;
}

Current Output:

---

The `DoubleDimension2d` message represents a dimension with **two** elements
(*width* and *height*) in double precision.

---

Expected Output:

---

The DoubleDimension2d message represents a dimension with two elements (width and height) in double precision.

---

Notice that this issue also describes the problem mentioned in my comment to #274 regarding line breaks.