Adding a free-text option to customise JVM

found that some setups will need to customise the JVM to work in different
environments (eg. https://confluence.atlassian.com/display/JIRAKB/How+to+Configure+an+Outbound+HTTP+and+HTTPS+Proxy+for+JIRA)

rather than provide an exhausting list of properties to add/customise, I felt
this should be freetext to allow addition of more properties.

A list of known properties from atlassian:
https://confluence.atlassian.com/display/JIRA/Setting+Properties+and+Options+on+Startup
Merge pull request #8 from preflightsiren/additional_jvm_options

Additional jvm options