Allow tox to pass through arguments

Passing through commandline arguments to commands allows to easily
configure what is run in environments. For instance, this allows to
specify what particular files to run linters against or what tests to
execute.

This change additionally moves arguments to linter commands into a
configuration constant.

Issue: HIC-213
Change-Id: Id6c38cd8376ef505dd372da1ee13edb289a10974
diff --git a/tox.ini b/tox.ini
index 6d5975b..13682f8 100644
--- a/tox.ini
+++ b/tox.ini
@@ -12,7 +12,7 @@
 deps =
     -rrequirements.txt
 commands =
-    python manage.py test
+    python manage.py test {posargs}
 
 # Test coverage
 [testenv:coverage]
@@ -32,18 +32,21 @@
     coverage html
 
 # Linters
+[linters]
+include_dirs = crashreports crashreport_stats hiccup
+
 [testenv:flake8]
 deps =
     -rrequirements-dev-flake8.txt
 commands =
-    flake8 crashreports crashreport_stats hiccup
+    flake8 {posargs: {[linters]include_dirs}}
 
 [testenv:pylint]
 deps =
     -rrequirements.txt
     -rrequirements-dev-pylint.txt
 commands =
-    pylint crashreports crashreport_stats hiccup
+    pylint {posargs: {[linters]include_dirs}}
 
 [testenv:linters]
 deps =