change default markdown tab width to 4 spaces

[Schlaefer]

People expect a one tab indent to be escaped content in markdown (see recent discussion in gitter), but with the current config set to 2 you actually need two tabs.

See tab_width note: Keep in mind that when the Markdown syntax spec says “four spaces or one tab”, it actually means “four spaces after tabs are expanded to spaces”. So setting tab_width to 8 will make parser treat a tab character as two levels of indentation.

[Schlaefer]

Closed with #233