Support omnilinter: ignore markers (fixes #34)

Omnilinter won't match lines with these markers, allowing to tune its
behavior from the code being checked.

.omnilinter.conf

@@ -10,12 +10,12 @@ rules:
   files: "src/*.rs"
   nomatch: "^//!"
 
-- title: TODO items
+- title: TODO items  # omnilinter: ignore
   files: "*.rs *.yml .omnilinter.conf"
-  match: TODO
-- title: FIXME items
+  match: TODO  # omnilinter: ignore
+- title: FIXME items  # omnilinter: ignore
   files: "*.rs *.yml .omnilinter.conf"
-  match: FIXME
+  match: FIXME  # omnilinter: ignore
 
 - title: omniparser does not exist  # I tend to make this misspelling occasionally for some reason
   files: "*.rs /[A-Z]*"

src/applier.rs

@@ -7,6 +7,8 @@ use std::fs;
 use std::path::Path;
 use walkdir::WalkDir;
 
+const IGNORE_MARKER: &str = "omnilinter: ignore";
+
 pub struct ApplierOptions {
     pub required_tags: HashSet<String>,
     pub ignored_tags: HashSet<String>,
@@ -45,7 +47,7 @@ fn apply_rule_to_path(loc: &FileMatchLocation, rule: &Rule, reporter: &mut dyn R
 
     if let Some(regex) = &rule.regex {
         for (nline, line) in text.lines().enumerate() {
-            if regex.is_match(line) {
+            if regex.is_match(line) && !line.contains(IGNORE_MARKER) {
                 reporter.report(
                     &MatchLocation::Line(LineMatchLocation::from_file(loc, nline + 1)),
                     &rule.title,

tests/suite.rs

@@ -245,3 +245,17 @@ fn tags() {
         )
         .assert_matches(vec![]);
 }
+
+#[test]
+fn ignore_marker() {
+    TestCase::new()
+        .add_file("a.py", "foo\nbar  # omnilinter: ignore")
+        .run_with_rule(
+            "
+            - title: test
+              files: '*.py'
+              match: 'foo|bar'
+            ",
+        )
+        .assert_matches(vec!["a.py:1"]);
+}