Add --client default arg

[Swiddis]

Newer versions of SBT support the experimental --client flag, which persists an SBT server in the background and reuses it instead of starting and tearing down a new server on every invocation. For complex projects with lots of packages/keys, this is a dramatic difference:

$ time sbt scalafmtCheckAll
...
sbt scalafmtCheckAll  36.77s user 2.98s system 570% cpu 6.962 total

$ time sbt scalafmtCheckAll --client
...
sbt scalafmtCheckAll --client  0.03s user 0.05s system 24% cpu 0.350 total

Supporting this argument in the precommit hooks will reduce commit times by several seconds for larger Scala projects.

Since the feature is still experimental and not supported on all SBT versions, I set it to disabled by default.

[Swiddis]

Tested on my dayjob's repo and it works as expected. Don't see any automated testing here.

[INFO] Initializing environment for https://github.com/Swiddis/scala-pre-commit-hooks.
[INFO] Installing environment for https://github.com/Swiddis/scala-pre-commit-hooks.
[INFO] Once installed this environment will be reused.
[INFO] This may take a few minutes...
Scala unused imports (+ fatal warnings)..................................Passed
scalafmt formatting fix..................................................Passed
[precommit 9f5f52b5] Test
 1 file changed, 4 insertions(+), 4 deletions(-)

[adamw]

Thank you! Yes, automated tests are lacking, though I'm not sure how they could work. But if you'd have an idea, I'll be happy to merge :)