refactor: support parallel execution

* parallel processing of multiple repositories
* parallel parsing of git history
* parallel template output
* configuration option for number of threads
* improved detection of exceeded size limits

Side effects of parallel execution are out-of-order output messages,
and larger potential overshoot in both processing and detecting when
limits are exceeded.  It will stop if it exceeds the limits, but it
can exceed them by more before stopping.

Processing of extremely large repositories is several times faster.

The order of commits in the history output might not match the output
of `git log`, since the order of commits in the log can change when
splitting it up into smaller pieces.

src/settings.rs

@@ -399,6 +399,7 @@ pub struct GitsySettings {
     pub paginate_history: Option<usize>,
     pub paginate_branches: Option<usize>,
     pub paginate_tags: Option<usize>,
+    pub threads: Option<usize>,
     pub limit_history: Option<usize>,
     pub limit_commits: Option<usize>,
     pub limit_branches: Option<usize>,