Support multiple formatters (radian-software#31)

Closes radian-software#31

This commit makes it so apheleia can run multiple formatters one after
the other and use the resultant output to format the current buffer.
This works somewhat like a pipeline. The output of one formatter becomes
the input to the next formatter until all formatters have run and then
an RCS patch is built from the resultant output and applied to the
current buffer.

Note: For convenience we internally represent the users configuration as
a list of formatters even when it may only be one. For example if the
user has configured `(python-mode . black)` in apheleia-mode-alist then
internally we interpret that as a formatter list of `(black)` instead of
`black` as we did previously.

CHANGELOG.md

@@ -4,6 +4,11 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog].
 
 ## Unreleased
+### Enhancements
+* Support multiple formatters ([#31]). You can now configure a list of
+  formatters for a major-mode in `apheleia-mode-alist` and they will
+  be run in sequence.
+
 ### Formatters
 * [ClangFormat](https://clang.llvm.org/docs/ClangFormat.html) for
   C/C++
@@ -29,6 +34,7 @@ The format is based on [Keep a Changelog].
 
 [#24]: https://github.com/raxod502/apheleia/pull/24
 [#30]: https://github.com/raxod502/apheleia/issues/30
+[#31]: https://github.com/raxod502/apheleia/issues/31
 [#32]: https://github.com/raxod502/apheleia/pull/32
 [#39]: https://github.com/raxod502/apheleia/issues/39
 [#48]: https://github.com/raxod502/apheleia/pull/48

README.md

@@ -83,10 +83,30 @@ variables:
       (setf (alist-get 'black apheleia-formatters)
             '("black" "--option" "..." "-"))
       ```
-
 * `apheleia-mode-alist`: Alist mapping major modes and filename
   regexps to names of formatters to use in those modes and files. See
   the docstring for more information.
+    * You can use this variable to configure multiple formatters for
+      the same buffer by setting the `cdr` of an entry to a list of
+      formatters to run instead of a single formatter. For example you
+      may want to run `isort` and `black` one after the other.
+
+      ```elisp
+      (setf (alist-get 'isort apheleia-formatters)
+            '("isort" "--stdout" "-"))
+      (setf (alist-get 'python-mode apheleia-mode-alist)
+            '(isort black))
+      ```
+
+      This will make apheleia run `isort` on the current buffer and then
+      `black` on the result of `isort` and then use the final output to
+      format the current buffer.
+
+      **Warn**: At the moment there's no smart or configurable error
+      handling in place. This means if one of the configured
+      formatters fail (for example if `isort` isn't installed) then
+      apheleia just doesn't format the buffer at all, even if `black`
+      is installed.
 * `apheleia-formatter`: Optional buffer-local variable specifying the
   formatter to use in this buffer. Overrides `apheleia-mode-alist`.