Clarify README, destroy by default and fix temp dir bug

I've changed three things. The readme, default behaviour and fixed a bug in temp dir location generation.

## README.md
I've added all defaults to the parameters listed in the readme.

## Default behaviour
Detroy is now enabled by default. This is the safest way to use ssh-keygen because it leaves no evidence by default. It also generates the lowest waste, which keeps the temp dir clean by default.

## Temp dir location generation
When location is not provided, ssh-keygen generates a temp path. This generation didn't split the temp directory and key directory name with a slash. I changed the code to use `path.join`.