proselint for mail files (#1037)

Add proselint for mail files

README.md

@@ -109,6 +109,7 @@ formatting.
 | LaTeX | [chktex](http://www.nongnu.org/chktex/), [lacheck](https://www.ctan.org/pkg/lacheck), [proselint](http://proselint.com/) |
 | LLVM | [llc](https://llvm.org/docs/CommandGuide/llc.html) |
 | Lua | [luacheck](https://github.com/mpeterv/luacheck) |
+| Mail | [proselint](http://proselint.com/) |
 | Markdown | [mdl](https://github.com/mivok/markdownlint), [proselint](http://proselint.com/), [vale](https://github.com/ValeLint/vale), [remark-lint](https://github.com/wooorm/remark-lint) !! |
 | MATLAB | [mlint](https://www.mathworks.com/help/matlab/ref/mlint.html) |
 | Nim | [nim check](https://nim-lang.org/docs/nimc.html) !! |

ale_linters/mail/proselint.vim

@@ -0,0 +1,9 @@
+" Author: Daniel M. Capella https://github.com/polyzen
+" Description: proselint for mail files
+
+call ale#linter#Define('mail', {
+\   'name': 'proselint',
+\   'executable': 'proselint',
+\   'command': 'proselint %t',
+\   'callback': 'ale#handlers#unix#HandleAsWarning',
+\})

doc/ale.txt

@@ -266,6 +266,7 @@ Notes:
 * LaTeX (tex): `chktex`, `lacheck`, `proselint`
 * LLVM: `llc`
 * Lua: `luacheck`
+* Mail: `proselint`
 * Markdown: `mdl`, `proselint`, `vale`, `remark-lint`
 * MATLAB: `mlint`
 * Nim: `nim check`!!