Enable spell check and word completion in markdown

vim/ftplugin/markdown.vim

@@ -1,2 +1,18 @@
 " set textwidth for automatic line wrap
 set tw=80
+set spell spelllang=en_us
+
+" disable real time linting, as it might be distracting
+let g:ale_lint_on_text_changed=0
+
+" complete english words
+setlocal dictionary+=/usr/share/dict/words
+" Remove this if you'd like to use fuzzy search
+call deoplete#custom#source(
+            \ 'dictionary', 'matchers', ['matcher_head'])
+" If dictionary is already sorted, no need to sort it again.
+call deoplete#custom#source(
+            \ 'dictionary', 'sorters', [])
+" Do not complete too short words
+call deoplete#custom#source(
+            \ 'dictionary', 'min_pattern_length', 4)