Improve pyrex cython linter.

Like many other linters, use variables for the executable and options
used by the linter.
By default, the linter now report every warnings as errors with
`--warning-errors`.
Also add include directory and set working directory to file directory.

ale_linters/pyrex/cython.vim

@@ -1,10 +1,27 @@
-" Author: w0rp <devw0rp@gmail.com>
+" Author: w0rp <devw0rp@gmail.com>,
+" Nicolas Pauss <https://github.com/nicopauss>
 " Description: cython syntax checking for cython files.
 
+call ale#Set('pyrex_cython_executable', 'cython')
+call ale#Set('pyrex_cython_options', '--warning-extra --warning-errors')
+
+function! ale_linters#pyrex#cython#GetExecutable(buffer) abort
+    return ale#Var(a:buffer, 'pyrex_cython_executable')
+endfunction
+
+function! ale_linters#pyrex#cython#GetCommand(buffer) abort
+    let l:local_dir = ale#Escape(fnamemodify(bufname(a:buffer), ':p:h'))
+
+    return ale#Escape(ale_linters#pyrex#cython#GetExecutable(a:buffer))
+    \   . ' --working ' . l:local_dir . ' --include-dir ' . l:local_dir
+    \   . ' ' . ale#Var(a:buffer, 'pyrex_cython_options')
+    \   . ' --output-file ' . g:ale#util#nul_file . ' %t'
+endfunction
+
 call ale#linter#Define('pyrex', {
 \   'name': 'cython',
 \   'output_stream': 'stderr',
-\   'executable': 'cython',
-\   'command': 'cython --warning-extra -o ' . g:ale#util#nul_file . ' %t',
+\   'executable_callback': 'ale_linters#pyrex#cython#GetExecutable',
+\   'command_callback': 'ale_linters#pyrex#cython#GetCommand',
 \   'callback': 'ale#handlers#unix#HandleAsError',
 \})