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>