Add if-exists clause to drop table

spec/avram/migrator/drop_table_statement_spec.cr

@@ -4,6 +4,13 @@ describe Avram::Migrator::DropTableStatement do
   it "can drop table" do
     statement = Avram::Migrator::DropTableStatement.new(:users).build
 
-    statement.should eq "DROP TABLE users"
+    statement.should eq "DROP TABLE users;"
+  end
+
+  context "IF EXISTS" do
+    it "adds the option to the table" do
+      statement = Avram::Migrator::DropTableStatement.new(:users, if_exists: true).build
+      statement.should eq "DROP TABLE IF EXISTS users;"
+    end
   end
 end

src/avram/migrator/drop_table_statement.cr

@@ -1,8 +1,8 @@
 class Avram::Migrator::DropTableStatement
-  def initialize(@table_name : TableName)
+  def initialize(@table_name : TableName, @if_exists : Bool = false)
   end
 
   def build
-    "DROP TABLE #{@table_name}"
+    "DROP TABLE #{@if_exists ? "IF EXISTS " : ""}#{@table_name};"
   end
 end

src/avram/migrator/statement_helpers.cr

@@ -14,8 +14,8 @@ module Avram::Migrator::StatementHelpers
     end
   end
 
-  def drop(table_name)
-    prepared_statements << Avram::Migrator::DropTableStatement.new(table_name).build
+  def drop(table_name, *, if_exists = false)
+    prepared_statements << Avram::Migrator::DropTableStatement.new(table_name, if_exists).build
   end
 
   macro alter(table_name, *, if_exists = false)