Make it easier to get FlexASIO debug logs

[dechamps]

While investigating #3 it became clear that FlexASIO could make it easier to get debug logs. ASIO host applications usually don't provide error details at all (such as which ASIO call failed and how), so we need to surface them through a side channel.

Right now the side channel that FlexASIO uses is OutputDebugString(). This is great for developers, but not so great for end users because it requires them to install a debugger (such as WinDbg) to get the logs.

FlexASIO should instead make it possible to get these logs in a more user-friendly way, such as logging to a file.

Related: #4