UPDATED: BuyTokens getting the token amount from a function instead of just multiplication. Useful for making Crowdsales with bonus periods

contracts/crowdsale/Crowdsale.sol

@@ -59,12 +59,16 @@ contract Crowdsale {
     return new MintableToken();
   }
 
-
   // fallback function can be used to buy tokens
   function () external payable {
     buyTokens(msg.sender);
   }
 
+  // Override this method to have a way to add business logic to your crowdsale when buying
+  function getTokenAmount(uint256 weiAmount) internal view returns(uint256) {
+    return weiAmount.mul(rate);
+  }
+
   // low level token purchase function
   function buyTokens(address beneficiary) public payable {
     require(beneficiary != address(0));
@@ -73,7 +77,7 @@ contract Crowdsale {
     uint256 weiAmount = msg.value;
 
     // calculate token amount to be created
-    uint256 tokens = weiAmount.mul(rate);
+    uint256 tokens = getTokenAmount(weiAmount);
 
     // update state
     weiRaised = weiRaised.add(weiAmount);