MdBook Variables preprocessor

A simple preprocessor for mdbook, that look for variable in double brackets and replace with some value that come from the book.toml

Example

SimpleFile.md: ```

something

a contentent with a variable {{name}} ```

book.toml ```toml

... all the basic detail first and then:

[preprocessor.variables.variables] name= "my wonderful name" ```

It does support looking for variable values in the environment variables behind a flag

Toml configured variable right now take precedence on environment variables. ```toml

[preprocessor.variables] use_env = true

[preprocessor.variables.variables] othervariablednotinenv= "value" ```

The implementation got a lot of inspiration and code from the mdbook links preprocessor an mdbook-plantuml.