Trim vim system commands (#69)

When vim executes a system command it gets the full string response, including the trailing newline character

This newline needs to be trimmed off the end of the return value before doing a string comparison with the value of the existing colorscheme

This prevents the focus handler resetting the colorscheme every time it's called, but rather just when an actual change has occurred

USAGE.md

@@ -218,7 +218,7 @@ local function get_tinty_theme()
   if vim.v.shell_error ~= 0 then
     return default_theme
   else
-    return theme_name
+    return vim.trim(theme_name)
   end
 end
 
@@ -258,7 +258,7 @@ function! GetTintyTheme()
   if v:shell_error != 0
     return g:default_theme
   else
-    return l:theme_name
+    return trim(l:theme_name)
   endif
 endfunction