feat: add simple config for iSearch

[candywater]

#15

add config feature and add 2 simple config for iSearch

will search config file by below order
(a) source file directory
(b) ~/.iSearch/iSearch.txt
(c) ~/.config/iSearch/iSearch.txt
(d) system variable ISEARCH_CONF

2 config parameters
DEFAULT_SAVE_DB_LEVEL, same as '请输入,放弃保存0，优先级(1~5)(默认为3)，6自定义\n>>> ', default is 3
SKIP_SAVE_DB_CONFIRM_MESSAGE can ignore above confirm message. default is false

if there is no config file, then will follow default settings

DEFAULT_CONFIG = {
    "skipSaveDbConfirmMessage": False,
    "defaultSaveDbLevel": "3"
}

---

#15

添加了config文件功能, 并添加两个配置项

会遵循以下优先级查询config文件
(a) 源代码目录
(b) ~/.iSearch/iSearch.txt
(c) ~/.config/iSearch/iSearch.txt
(d) 系统环境变量 ISEARCH_CONF

新增配置变量
DEFAULT_SAVE_DB_LEVEL, 定义右边输入的文字， '请输入,放弃保存0，优先级(1~5)(默认为3)，6自定义\n>>> ', 默认为3
SKIP_SAVE_DB_CONFIRM_MESSAGE 可以忽略（不输出）上述的提示信息，默认是False（默认输出）

如果查询不到配置文件，将采取以下配置

DEFAULT_CONFIG = {
    "skipSaveDbConfirmMessage": False,
    "defaultSaveDbLevel": "3"
}

[candywater]

abort this PR.
I'll do a total rework from v1.0.2.
once I finished work, if dev owner wants a PR then I'll raise one.