Allow using separate converters for each argument

When converting an argument in a position, if there is a function
in the same position in provided converer array, use
it as a converter function.

This allows using an array of functions as the value of `convert` property,
with each function applied to corresponding argument of a multi-argument option.

Ref #14.

spec/convert_spec.lua

@@ -13,6 +13,24 @@ describe("tests related to converters", function()
       assert.same({numbers = {1, 2, 500}}, args)
    end)
 
+   it("accepts an array of converters", function()
+      local function tocoords(str)
+         local x, y = str:match("^([^,]*),([^,]*)$")
+         x = tonumber(x)
+         y = tonumber(y)
+         return x and y and {x, y}
+      end
+
+      local parser = Parser()
+      parser:option "-c --circle" {
+         convert = {tonumber, tocoords},
+         args = 2
+      }
+
+      local args = parser:parse{"-c", "123", "456,567"}
+      assert.same({circle = {123, {456, 567}}}, args)
+   end)
+
    it("converts arguments using mapping", function()
       local choice = {
          foo = 1,

src/argparse.lua

@@ -781,7 +781,7 @@ function ElementState:error(fmt, ...)
    self.state:error(fmt, ...)
 end
 
-function ElementState:convert(argument)
+function ElementState:convert(argument, index)
    local converter = self.element._convert
 
    if converter then
@@ -789,6 +789,8 @@ function ElementState:convert(argument)
 
       if type(converter) == "function" then
          ok, err = converter(argument)
+      elseif type(converter[index]) == "function" then
+         ok, err = converter[index](argument)
       else
          ok = converter[argument]
       end
@@ -844,7 +846,7 @@ function ElementState:invoke(alias)
 end
 
 function ElementState:pass(argument)
-   argument = self:convert(argument)
+   argument = self:convert(argument, #self.args + 1)
    table.insert(self.args, argument)
 
    if #self.args >= self.element._maxargs then