Add background file logging (#2762)

## Issue Addressed

Closes #1996 

## Proposed Changes

Run a second `Logger` via `sloggers` which logs to a file in the background with:
- separate `debug-level` for background and terminal logging
- the ability to limit log size
- rotation through a customizable number of log files
- an option to compress old log files (`.gz` format)

Add the following new CLI flags:
- `--logfile-debug-level`: The debug level of the log files
- `--logfile-max-size`: The maximum size of each log file
- `--logfile-max-number`: The number of old log files to store
- `--logfile-compress`: Whether to compress old log files

By default background logging uses the `debug` log level and saves logfiles to:
- Beacon Node:  `$HOME/.lighthouse/$network/beacon/logs/beacon.log`
- Validator Client:  `$HOME/.lighthouse/$network/validators/logs/validator.log`

Or, when using the `--datadir` flag:
`$datadir/beacon/logs/beacon.log` and `$datadir/validators/logs/validator.log`

Once rotated, old logs are stored like so: `beacon.log.1`, `beacon.log.2` etc. 
> Note: `beacon.log.1` is always newer than `beacon.log.2`.

## Additional Info

Currently the default value of `--logfile-max-size` is 200 (MB) and `--logfile-max-number` is 5.
This means that the maximum storage space that the logs will take up by default is 1.2GB. 
(200MB x 5 from old log files + <200MB the current logfile being written to)
Happy to adjust these default values to whatever people think is appropriate. 

It's also worth noting that when logging to a file, we lose our custom `slog` formatting. This means the logfile logs look like this:
```
Oct 27 16:02:50.305 INFO Lighthouse started, version: Lighthouse/v2.0.1-8edd9d4+, module: lighthouse:413
Oct 27 16:02:50.305 INFO Configured for network, name: prater, module: lighthouse:414
```

common/directory/src/lib.rs

@@ -40,6 +40,19 @@ pub fn ensure_dir_exists<P: AsRef<Path>>(path: P) -> Result<(), String> {
     Ok(())
 }
 
+/// If `arg` is in `matches`, parses the value as a path.
+///
+/// Otherwise, attempts to find the default directory for the `testnet` from the `matches`.
+pub fn parse_path_or_default(matches: &ArgMatches, arg: &'static str) -> Result<PathBuf, String> {
+    clap_utils::parse_path_with_default_in_home_dir(
+        matches,
+        arg,
+        PathBuf::new()
+            .join(DEFAULT_ROOT_DIR)
+            .join(get_network_dir(matches)),
+    )
+}
+
 /// If `arg` is in `matches`, parses the value as a path.
 ///
 /// Otherwise, attempts to find the default directory for the `testnet` from the `matches`