add filter operators

The following filter expressions are now additionally valid:

```
--filter @tag <=VALUE
--filter @tag <VALUE
--filter @tag >=VALUE
--filter @tag >VALUE
```

Note that negation needs some re-work. Currently, only `--filter !@tag
<VALUE` is valid, and `--filter @tag !<VALUE` is not. The first version
may not behave as you would expect it.
A tracking issue has been created.

Note that without typing tag values, all comparisons perform
lexicographically on `String`s.

1 files changed, 14 insertions, 1 deletions

diff --git a/app/Issue/Tag.hs b/app/Issue/Tag.hs
index 8b6d6d4..85636b5 100644
--- a/app/Issue/Tag.hs
+++ b/app/Issue/Tag.hs
@@ -1,4 +1,11 @@
-module Issue.Tag (Tag (..), extractTags, internalTags) where
+module Issue.Tag
+  ( Tag (..),
+    extractTags,
+    internalTags,
+    tagKey,
+    tagValue,
+  )
+where
 
 import Data.Binary (Binary)
 import Data.Maybe (catMaybes)
@@ -10,6 +17,12 @@ import Issue.Provenance (Provenance (..))
 
 data Tag = Tag Text (Maybe Text) deriving (Show, Generic, Binary)
 
+tagKey :: Tag -> Text
+tagKey (Tag k _) = k
+
+tagValue :: Tag -> Maybe Text
+tagValue (Tag _ v) = v
+
 extractTags :: Text -> [Tag]
 extractTags =
   catMaybes