add load_param_mem(), which load plain text params from memory buf.

[xinyu391]

虽然有load_param(const unsigned char*& mem)，从内存加载 bin格式的params，但是bin格式，只能通过layer index 指定输入输出，不同模型的layer index 不一样，但通过plain text 格式的params，则可以定义特定的输入输出名字。
虽然有 fmemopen()函数可以将一块内存映射位FILE*,就可以套用load_param(FILE* fp)， 但是fmemopen函数只能在Android6.0+上使用。
所以加了这两个函数，函数内容和load_param(FILE* fp);几乎一样，只是通过宏来更改位从内存字符串中sscanf数据。

[tencent-adm]

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[xinyu391]

咋回事啊，一直pending中

[nihui]

咋回事啊，一直pending中

注册下CLA ...

[xinyu391]

注册了也签名了，但是我github邮箱，和git commit里面的不一样，在github中增加邮箱后，点击上面的 “recheck“ ，出现404.我尝试重新 pull request下