allow formatting of bamboo.repository.revision.number

Hi,

We are using ${bamboo.repository.revision.number} to embed within our application. We are also using mercurial. This generates a LONG number (which can be ugly for clients to see), for example:

8f0c90a4c158c0d66bec4cb390946c025891576a887c

It would be good, if we could restrict that to something like:

${bamboo.repository.revision.number|13}

8f0c90a4c158

I think this would come in useful!