Add possibility to style the individual prompt placeholders

[sbreakey]

Added the possibility to style the individual prompt placeholders, since I use a few of the placeholders but want the hostname to stand out a bit more. Uses the prompt_toolkit's HTML() method, as splitting the prompt into a list of tuples seemed overkill.

Ideally I wanted to be able to style the prompt based on a regex or some sort of string matching, so that I could style it as red when I am querying a production database for example, just as a hint not to do anything stupid :) ... but I went for the easier option.

[ghost]

All CLA requirements met.