Add flag 'log-color' preserving color of log redirected to file. (#3538)

Add flag 'log-color' which preserves colors of log when stdout is redirected to a file.

This is my first lighthouse PR, please let me know if I'm not following contribution guidelines, I welcome meta-feeback (feedback on git commit messages, git branch naming, and the contents of the description of this PR.)

## Issue Addressed

Solves https://github.com/sigp/lighthouse/issues/3527

## Proposed Changes

Adding a flag which enables log color preserving when stdout is redirected to a file.

### Usage
Below I demonstrate current behaviour (without using the new flag) and the new behaviur (when using new flag).

In the screenshot below, I have to panes, one on top running `lighthouse` which redirects to file `~/Desktop/test.log` and one pane in the bottom which runs `tail -f ~/Desktop/test.log`.

#### Current behaviour
```sh
lighthouse --network prater vc |& tee -a ~/Desktop/test.log
```

**Result is no colors**

<img width="1624" alt="current" src="https://user-images.githubusercontent.com/864410/188258226-bfcf8271-4c9e-474c-848e-ac92a60df25c.png">


#### New behaviour
```sh
lighthouse --network prater vc --log-color |& tee -a ~/Desktop/test.log
```

**Result is colors** 🔴🟢🔵🟡

<img width="1624" alt="new" src="https://user-images.githubusercontent.com/864410/188258223-7d9ecf09-92c8-4cba-8f24-bd4d88fc0353.png">

## Additional Info

I chose American spelling of "color" instead of Brittish "colour' since that was aligned with `slog`'s API - method`force_color()`, let me know if you prefer spelling "colour" instead. I also chose to let it be an arg not taking any argument, just like `logfile-compress` flag, rather than having to write `--log-color true`.

lighthouse/environment/src/lib.rs

@@ -47,6 +47,7 @@ pub struct LoggerConfig<'a> {
     pub debug_level: &'a str,
     pub logfile_debug_level: &'a str,
     pub log_format: Option<&'a str>,
+    pub log_color: bool,
     pub max_log_size: u64,
     pub max_log_number: usize,
     pub compression: bool,
@@ -139,7 +140,13 @@ impl<E: EthSpec> EnvironmentBuilder<E> {
                 _ => return Err("Logging format provided is not supported".to_string()),
             }
         } else {
-            let stdout_decorator = slog_term::TermDecorator::new().build();
+            let stdout_decorator_builder = slog_term::TermDecorator::new();
+            let stdout_decorator = if config.log_color {
+                stdout_decorator_builder.force_color()
+            } else {
+                stdout_decorator_builder
+            }
+            .build();
             let stdout_decorator =
                 logging::AlignedTermDecorator::new(stdout_decorator, logging::MAX_MESSAGE_WIDTH);
             let stdout_drain = slog_term::FullFormat::new(stdout_decorator).build().fuse();