safemap 2.0.0-nullsafety.0

Safely get value with Type from JSON map. Will return null value when value was wrong.

[2.0.0-nullsafety.0].

• Add nullsafety

[1.2.3].

• toDouble getter can auto transfer int to double now(but not return null value).

[1.2.2].

• Add toInt getter to transfer String to int.
• Add toDouble getter to transfer String to double.

[1.2.1].

• Fix list bug.

[1.0.3].

• Fix bug.

[1.0.2].

• 不再依赖Flutter

[1.0.1].

• 添加了int与double的直接取值.
• 添加了Example.

[1.0.0].

• init.