quality of life improvements

[nilslice]

1. errors
   error formatting could be improved vs. setting a []const u8.. more like https://github.com/ziglang/zig/blob/master/lib/std/fmt.zig#L83

e.g.

try plugin.allocError("some error {s}", .{ err });

(this probably applies generally, wherever we take []const u8 for things to signal info back to the host.. e.g. logging perhaps)

2. use new std HTTP types
   such as the HTTP methods and other items where possible

thank you to @ethanholz for the inspiration and user report on Twitch! (https://www.twitch.tv/ninja_tron)

3. make LogLevel public
   then its more flexible to write wrapper logging functions

[nilslice]

Sorry this took so long to get around to! But there's a PR open at #38

[ethanholz]

These looks cool! Great work!