Adds the `tsContentHeader` and `tsContentFooter` settings

[cmaster11]

Adds the tsContentHeader and tsContentFooter settings.

They let you prepend/append some code to every generated interface. This can be useful when you generate documentation automatically and you want to import full code blocks in your docs by defining begin/end markers of the various blocks.

E.g.

tsContentHeader: type => `// [block "${type.interfaceOrTypeName}" begin]`,
tsContentFooter: type => `// [block "${type.interfaceOrTypeName}" end]`

// [block "Other" begin]
export interface Other {
  other?: string;
}
// [block "Other" end]

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (8b4fb10) 98.67% compared to head (fa34f6a) 98.68%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #398 +/- ## ======================================= Coverage 98.67% 98.68% ======================================= Files 9 9 Lines 528 532 +4 Branches 204 206 +2 ======================================= + Hits 521 525 +4 Misses 7 7 ```

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