New CodeMirror, Better themeability

This version contains:

• Updates on CodeMirror (syntax highlighting in plaintext editor)
• Request data made available to Velocity engine, so that themes can access request headers, parameters, etc allowing more customization

Maven:

<dependency>
            <groupId>com.github.tcnh</groupId>
            <artifactId>fitnesse</artifactId>
            <version>20181214</version>
</dependency>