Feat: add variable and fix response

[haotianzhu]

Check List

• [x] Fix #48 (write the relative issue number)

• [x] Branch is up to date with master

• [x] Now it's a good time for reviewing

• [x] Branch should have prefix either "feature/" or "fix/"

Comment

• [x] I add two types of variable : fix-single and fix-list

fix-single is a fixed single variable fix-list is a list of fixed single variables

the single variable contains: name, type, name of a variable is the pattern of variable which is unique.

the type of the variable contains name of type and some parameters

For example

    "variables": [
      {
        "name": "v1",
        "type": {
          "name": "fix-single",
          "value": "fixed string"
        }
      },
      {
        "name": "v2",
        "type": {
          "name": "fix-single",
          "value": "fixed string2"
        }
      },
      {
        "name": "v3",
        "type": {
          "values": [
            "1",
            "2",
            "3",
            4
          ],
          "name": "fix-list"
        }
      }
    ],

each type of variable has a generating function to generate a variable for question- attempt

50

• [x] remove question.weight and change response.weight to mark

Put 'closes #.' in your comment to auto-close the issue that your PR fixes (if such).