Allow additional arguments to be passed to the MSBuild plugin

It is useful to be able to pass additional arguments (or override the default ones) by appending them to the end of the command line.

(e.g. /v:d)

This would help with the general case, though when the plugin is remodelled in the other issue, it would be good to cover the most specific ones with dedicated fields like <tasks>, linking debug level to the plugin log level, etc.