Spoofax/928: allows users to choose their own JVM options.

This pull requests allows users to choose their own JVM options:

- ESV support, e.g.:   `jvm opts:      -server -Xss8m -Xmx1024m` (Format: `-X[ss|mx]<size>[g|G|m|M|k|K]`)
- At runtime, it takes all the languages in the registry into account to calculate the highest Xss and the highest Xmx. If the highest Xss/Xmx is higher than the current Xss/Xmx, a configuration warning is generated.
- If a plugin specifies no JVM options, then the default options for Spoofax are used (`-server -Xss8m -Xmx1024m`)
- If there is only one language in the registry, the configuration warning is now named `YourLang configuration warning` instead of `Spoofax configuration warning`.

Note: Only -server, Xss and Xmx JVM options are currently supported.