Add selectable market benchmark in MWRR/TWRR performance

It would be great to be able to choose the market benchmark used in the MWRR/TWRR portfolio performance calculations. Currently the FTSE All-World ETF (£VWRL) is used as a benchmark which is fine.

But it would be awesome to choose other benchmarks such as the S&P500 or even some more niche markets. For example, I invest a lot in British small caps listed on AIM and it would be nice to compare my performance against the FTSE AIM All-Share.

A neat little dropdown to change the benchmark would be a great addition.