Add support for custom section anchors (#1179)

This allows to have stable anchors for groups, even if the set of
groups in the documentation is altered.

The syntax for setting the anchor of a group is

-- * Group name #desiredAnchor#

Which will produce an html anchor of the form '#g:desiredAnchor'

Co-authored-by: Iñaki García Etxebarria <git@inaki.blueleaf.cc>

1 files changed, 8 insertions, 0 deletions

diff --git a/html-test/src/SectionLabels.hs b/html-test/src/SectionLabels.hs
new file mode 100644
index 00000000..560bafa4
--- /dev/null
+++ b/html-test/src/SectionLabels.hs
@@ -0,0 +1,8 @@
+module SectionLabels
+  (
+    -- * Section heading#custom#
+    n
+  ) where
+
+n :: Int
+n = 3