Ability to pass a custom web.xml file

This allows the user to setup a *default* server setup for when the particular WAR file that is being run does not have its own WEB-INF.

In our particular use case, we want a generic command in CommandBox to start a server that is preconfigured with tuckey rewrite. We do not want each application to have to define that and so we fix it by supplying our own web.xml to define the filters and also supplying the jar file for the rewrite engine.