Improved help option behaviour

* Help option is only created once per parser, several :prepare() invocations do not create extra options.
* In :add_help(foo), foo is passed to the help option overriding default name, see spec/help_spec.lua @ 24

spec/help_spec.lua

@@ -11,6 +11,27 @@ describe("tests related to help message generation", function()
       }, "\r\n"), parser:prepare():get_help())
    end)
 
+   it("does not create extra help options when :prepare is called several times", function()
+      local parser = argparse.parser "foo"
+      assert.equal(table.concat({
+         "Usage: foo [-h]",
+         "",
+         "Options: ",
+         "   -h, --help            Show this help message and exit. "
+      }, "\r\n"), parser:prepare():prepare():get_help())
+   end)
+
+   it("uses custom help option", function()
+      local parser = argparse.parser "foo"
+         :add_help {aliases = {"\\?"}}
+      assert.equal(table.concat({
+         "Usage: foo [\\?]",
+         "",
+         "Options: ",
+         "   \\?                    Show this help message and exit. "
+      }, "\r\n"), parser:prepare():get_help())
+   end)
+
    it("creates correct help message for arguments", function()
       local parser = argparse.parser "foo"
       parser:argument "first"

src/argparse.lua

@@ -222,6 +222,7 @@ function Option:make_target()
    end
 
    self._target = self._target or self._aliases[1]:sub(2)
+   self._name = self._name or self._aliases[1]
 end
 
 function Parser:argument(...)
@@ -249,13 +250,20 @@ function Parser:command(...)
 end
 
 function Parser:prepare()
-   if self._add_help then
-      self:flag "-h" "--help"
-         :description "Show this help message and exit. "
+   if self._add_help and not self._help_option then
+      self._help_option = self:flag(self._add_help)
          :action(function()
             io.stdout:write(self:get_help() .. "\r\n")
             os.exit(0)
          end)
+
+      if #self._help_option._aliases == 0 then
+         self._help_option "-h" "--help"
+      end
+
+      if not self._help_option._description then
+         self._help_option:description "Show this help message and exit. "
+      end
    end
 
    for _, elements in ipairs{self._arguments, self._options} do