[GNC] New Balsheet (and P&L report)

[Christopher Lam]

Seeking beta testers.

https://github.com/christopherlam/gnucash/tree/maint-balsheet-pnl

Or, anyone with v3.2 onwards can copy the file directly into the build's 
standard-reports folder: 
https://raw.githubusercontent.com/christopherlam/gnucash/maint-balsheet-pnl/gnucash/report/standard-reports/balsheet-pnl.scm

Adds new balsheet and income statement (i.e. Profit&Loss) report. I 
still have further ideas in the pipeline, just wish to check accuracy of 
amounts produced. Not all options have been implemented.

C