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.

行数が表示されているエディター下部のステータス領域から始めます。カウンターがすべての単語を含むのか、それともコードブロックとURLを除外した純粋なテキストのみを含むのかを判断し、その後、行数の横に単語数を追加し、ノートの変更に応じて更新されることを確認します。

索引モデルが issue の本文から書いたものです。