add setting to hide the UI header

[rasmusq]

The header is the following text at the top of the window:

mason.nvim press g? for help https://github.com/williamboman/mason.nvim

For some, it is a little much. Being able to hide it would make it easier to have visual cohesion between other floating popups like Lazy.nvim.

I have added a simple boolean setting 'show_header'. When it is true (the default value), everything acts as it usually does. When it is false, the header is replaced with a single blank line acting as a margin.

It is a very simple setting, and it is my first PR contribution to a public project, so suggestions and corrections are expected.