Add scaffolding for an inline tag processor

This is just a simple test to extract the inline tags, it hasn't been
hooked up to anything.

Related to #44

lib/core/processors/inline_tags.dart

@@ -0,0 +1,28 @@
+import 'package:gitjournal/core/note.dart';
+
+class InlineTagsProcessor {
+  // FIXME: Make me configurable
+  final List<String> tagPrefixes = ['#'];
+
+  void process(Note note) {}
+
+  Set<String> extractTags(String text) {
+    var tags = <String>{};
+
+    for (var prefix in tagPrefixes) {
+      var regexp = RegExp(r'(^|\s)' + prefix + r'([^ ]+)(\s|$)');
+      var matches = regexp.allMatches(text);
+      for (var match in matches) {
+        var tag = match.group(2);
+
+        if (tag.endsWith('.') || tag.endsWith('!') || tag.endsWith('?')) {
+          tag = tag.substring(0, tag.length - 1);
+        }
+
+        tags.add(tag);
+      }
+    }
+
+    return tags;
+  }
+}

test/processors/inline_tags_test.dart

@@ -0,0 +1,28 @@
+import 'package:test/test.dart';
+
+import 'package:gitjournal/core/processors/inline_tags.dart';
+
+void main() {
+  test('Should parse simple tags', () {
+    var body = "#hello Hi\nthere how#are you #doing now? #dog";
+
+    var p = InlineTagsProcessor();
+    var tags = p.extractTags(body);
+
+    expect(tags, {'hello', 'doing', 'dog'});
+  });
+
+  test('Ignore . at the end of a tag', () {
+    var body = "Hi there #tag.";
+
+    var p = InlineTagsProcessor();
+    var tags = p.extractTags(body);
+
+    expect(tags, {'tag'});
+  });
+
+  // #a#b should be counted as two tags
+  // + should work as a prefix
+  // @ should work as a prefix
+  // test for tags with non-ascii words
+}