add custom extension support for importing source file

[KEINOS]

This PR features issue #286 by implementing tengo.Compiler.SetImportFileExt() method which allows specifying custom extension names.

This feature aims for "tengo" as a library usage and should not affect the "/cmd/tengo" itself.

func main(){
...
	c := tengo.NewCompiler(srcFile, nil, nil, modules, nil)
	c.EnableFileImport(true)
	c.SetImportDir(filepath.Dir(pathFileSource))
+	err := c.SetImportFileExt(".tengo", ".mshk") // It will search *.tengo and *.mshk script files
...
}

• Related issue
  • Can tengo module support changing script suffix？ #286
  • QUESTION: file extension for Tengo script #332

---

• EDIT: Previously, in the first commit, c.SetImportExt(".mshk") searched for both *.tengo and *.mshk (*.tengo by default) with no input validation.
  But currently it needs to explicitly define as c.SetImportFileExt(".tengo", ".mshk") for both file extensions. And returns an error if any given args are invalid as well.

[d5]

Thanks for the PR. I will have to look at the code later tonight, but can we change the function signature to something like:

SetImportFileExt(...string)

and it should replace the entire array instead of appending.

[KEINOS]

@d5
can we change the function signature

ok.

- func (c *Compiler) SetImportExt(ext string) {
+ func (c *Compiler) SetImportFileExt(exts ...string) {

it should replace the entire array instead of appending.

roger roger. (￣^￣)ゞ

[KEINOS]

I wonder if we have to document it but couldn't determine where. Is the godoc enough?

[d5]

we don't have documentation for that particular function, but maybe we can add a small note somewhere under here https://github.com/d5/tengo/blob/master/docs/tutorial.md#modules?

[KEINOS]

@d5
maybe we can add a small note somewhere under here

Done. How about it? Is it clear enough?

[geseq]

LGTM. Looks much better overall