Feature request: Word Count

Hi there,

Ive been using HackMD on my own server for 2 weeks right now and love it.

Everything seems to be here in term of text management except one little thing that is quite useful when writing papers: a word counter.

Such counter provides useful information about the complexity of what you are writing as well as let you respect a requirement when you need it.

Such counter can be added next to the line count at the bottom of the editor:

There is 2 way to do it as I see it:
- Count every words and create the counter
- Count every pure text words (do not include code block, underlying url in images and links, ...)

The second way seems more academic as codeblock, images and all are not considered part of the text but addition to it. A discussion about it is available here: https://github.com/MacDownApp/macdown/issues/244.

Comienza en el área de estado inferior del editor, donde se muestra el recuento de líneas. Decide si el contador incluye todas las palabras o solo texto puro, excluyendo los bloques de código y las URL, después añade el recuento de palabras junto al recuento de líneas y verifica que se actualice cuando cambie la nota.

Escrito por el modelo de indexación a partir del texto del issue.