config options for log format and color usage

We want to be able to disable colors for complement logs (since they
are likely to be opened in a text editor). There's no pressing need for
alternative log formats, but I'm interested in whether the 'pretty'
format will be easier for debugging.

I chose to add 'log_*' options rather than making a separate 'log'
section for now. There's been some discussion about trying to separate
the tracing/logging stuff into more structured sections, but that can
happen later.

Cargo.lock

@@ -3214,6 +3214,16 @@ dependencies = [
  "web-time",
 ]
 
+[[package]]
+name = "tracing-serde"
+version = "0.1.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bc6b213177105856957181934e4920de57730fc69bf42c37ee5bb664d406d9e1"
+dependencies = [
+ "serde",
+ "tracing-core",
+]
+
 [[package]]
 name = "tracing-subscriber"
 version = "0.3.18"
@@ -3224,12 +3234,15 @@ dependencies = [
  "nu-ansi-term",
  "once_cell",
  "regex",
+ "serde",
+ "serde_json",
  "sharded-slab",
  "smallvec",
  "thread_local",
  "tracing",
  "tracing-core",
  "tracing-log",
+ "tracing-serde",
 ]
 
 [[package]]