Merge in changes from PR 9 (@lkhphuc)

## Details

Merges in the default headings change from:
#9 by
@lkhphuc.

By adding an extra space certains font / terminal configurations
will be able to render a larger symbol.

Configurations which cannot take advantage of the additional space
will see no change.

Author: MeanderingProgrammer

Diff for: README.md

@@ -79,7 +79,7 @@ require('render-markdown').setup({
     -- modes will be uneffected by this plugin
     render_modes = { 'n', 'c' },
     -- Characters that will replace the # at the start of headings
-    headings = { '󰲡', '󰲣', '󰲥', '󰲧', '󰲩', '󰲫' },
+    headings = { '󰲡 ', '󰲣 ', '󰲥 ', '󰲧 ', '󰲩 ', '󰲫 ' },
     -- Character to use for the bullet point in lists
     bullet = '○',
     -- Character that will replace the > at the start of block quotes

Diff for: lua/render-markdown/init.lua

@@ -59,7 +59,7 @@ function M.setup(opts)
         ]],
         file_types = { 'markdown' },
         render_modes = { 'n', 'c' },
-        headings = { '󰲡', '󰲣', '󰲥', '󰲧', '󰲩', '󰲫' },
+        headings = { '󰲡 ', '󰲣 ', '󰲥 ', '󰲧 ', '󰲩 ', '󰲫 ' },
         bullet = '○',
         quote = '┃',
         highlights = {