Version: add plugin cmd option to print the version and exit

[wenchy]

• close Version: add plugin cmd option to print version #103
• improve security and readability of index package (https://github.com/tableauio/loader/tree/master/internal/index)

Problem

message TaskConf {
  option (tableau.worksheet) = {name:"TaskConf" index:"ActivityID<Goal" index:"ID>"};

  map<int64, Task> task_map = 1 [(tableau.field) = {key:"ID" layout:LAYOUT_VERTICAL}];
  message Task {
    int64 id = 1 [(tableau.field) = {name:"ID"}];
    // ...
  }
}

index panic problem: