Initial support for user-defined rules

This is the initial commit adding support for user-defined rules. It's a
rather large commit, but I decided to keep a few things out to
still try to keep the size manageable.

In particular, there will be follow-up commits adding more tests,
documentation and validation checks as well as general cleanup.

What is present in the current commit:
- Support for the 'extra-path' parameter that specifies where to search
  for user-defined rules
- A new user_rules.py module that implements the business logic for
  finding user-defined rule classes
- Example user-defined rules in examples/my_commit_rules.py and
  examples/my_line_rules.py
- A new DirectoryOption class for specifying directory paths as rule
  options
- ./run_tests.sh now print py.test warnings

1 files changed, 45 insertions, 0 deletions

diff --git a/examples/my_line_rules.py b/examples/my_line_rules.py
new file mode 100644
index 0000000..e5dda59
--- /dev/null
+++ b/examples/my_line_rules.py
@@ -0,0 +1,45 @@
+from gitlint.rules import LineRule, RuleViolation, CommitMessageTitle
+from gitlint.options import ListOption
+
+"""
+The SpecialChars class below is an example of a user-defined LineRule. Line rules are gitlint rules that only act on a
+single line at once. Once the rule is discovered, gitlint will automatically take care of applying this rule
+against each line of the commit message title or body (whether it is applied to the title or body is determined by the
+`target` attribute of the class).
+
+A LineRule contrasts with a CommitRule (see examples/my_commit_rules.py) in that a commit rule is only applied once on
+an entire commit. This allows commit rules to implement more complex checks that span multiple lines and/or checks
+that should only be done once per gitlint run.
+
+While every LineRule can be implemented as a CommitRule, it's usually easier and more concise to go with a LineRule if
+that fits your needs.
+"""
+
+
+class SpecialChars(LineRule):
+    """ This rule will enforce that the commit message title does not contain any of the following characters:
+        $^%@!*() """
+
+    # A rule MUST have a human friendly name
+    name = "title-no-special-chars"
+
+    # A rule MUST have an *unique* id, we recommend starting with UL (for User-defined Line-rule), but this can
+    # really be anything.
+    id = "UL1"
+
+    # A line-rule MUST have a target (not required for CommitRules).
+    target = CommitMessageTitle
+
+    # A rule MAY have an option_spec if its behavior should be configurable.
+    options_spec = [ListOption('special-chars', ['$', '^', '%', '@', '!', '*', '(', ')'],
+                               "Comma separated list of characters that should not occur in the title")]
+
+    def validate(self, line, _commit):
+        violations = []
+        # options can be accessed by looking them up by their name in self.options
+        for char in self.options['special-chars'].value:
+            if char in line:
+                violation = RuleViolation(self.id, "Title contains the special character '{}'".format(char), line)
+                violations.append(violation)
+
+        return violations