Alias ps1 filetype to powershell

Rather than requiring users to alias ps1 to powershell themselves,
include it in s:default_ale_linter_aliases.  Since [vim-ps1] is a
popular (the only?) PowerShell ftplugin and there do not appear to be
any other uses of ft=ps1 on vim.org, this seems like a safe and
reasonable default.

[vim-ps1]: http://www.vim.org/scripts/script.php?script_id=1327

Signed-off-by: Kevin Locke <kevin@kevinlocke.name>

doc/ale.txt

@@ -1188,6 +1188,7 @@ g:ale_linter_aliases                                     *g:ale_linter_aliases*
   \   'csh': 'sh',
   \   'javascriptreact': ['javascript', 'jsx'],
   \   'plaintex': 'tex',
+  \   'ps1': 'powershell',
   \   'rmarkdown': 'r',
   \   'rmd': 'r',
   \   'systemverilog': 'verilog',