Automatically associate master slide with special names

[k0kubun]

Another interface of md2key

In addition to current interface, the new way to specify master slides is added.

When you name master slides as "cover", "h1", "h2", "h3", "h4" or "h5", md2key will use the master slides for the first slide, slides with #, ##, ###, #### or #####.

Demo

example.md

# The cover
"cover" is used

# Header 1: "h1" is used

## Header 2
- "h2" is used

### Header 3
- "h3".

## Header 2 again
- "h2".

[amatsuda]

💮