logger: drop env_logger

This commit updates the `log` crate to 0.4 and drops the dependency on
env_logger. In particular, the latest version of env_logger brings in
additional non-optional dependencies such as chrono that I don't think is
worth including into ripgrep.

It turns out ripgrep doesn't need any fancy logging. We just need a concept
of log levels and the ability to print to stderr. Therefore, we just roll
our own super simple logger.

This update is motivated by the persistent configuration task. In
particular, we need the ability to toggle the global log level more than
once, and this doesn't appear to be possible with older versions of the
log crate.

globset/Cargo.toml

@@ -21,7 +21,7 @@ bench = false
 [dependencies]
 aho-corasick = "0.6.0"
 fnv = "1.0"
-log = "0.3"
+log = "0.4"
 memchr = "2"
 regex = "0.2.1"