Accept 'preview' as an install parameter.

Testing type of parameter to make sure string is tested as string and
dict as dict. Type information can be found with `:help type`

autoload/coc/powershell.vim

@@ -7,16 +7,25 @@ let s:is_vim         = !has('nvim')
 let s:install_script = s:root.'/install.ps1'
 
 function! coc#powershell#install(...)
-    if(exists('a:1.preview') && a:1.preview)
-        let s:flags = '-AllowPreRelease'
-    else
-        let s:flags = ''
+    let s:flags = ''
+    if(exists('a:1'))
+        let paramType = type(a:1)
+        " Type 1 seems to be a string...
+        if(paramType == 1)
+            if(a:1 == 'preview')
+                let s:flags = '-AllowPreRelease'
+            endif
+        " Type 4 is a dictionary...
+        elseif(s:paramType == 4)
+            if(exists('a:1.preview') && a:1.preview)
+                let s:flags = '-AllowPreRelease'
+            endif
+        endif
     endif
-    let s:powershell_executable = ''
+    let s:powershell_executable = 'pwsh'
     if(exists('a:1.powershellExecutable'))
         let s:powershell_executable = a:1.powershellExecutable
     else
-        let s:powershell_executable = 'pwsh'
         if(s:is_win)
             let s:powershell_executable = "powershell"
             if(executable("pwsh"))