chore: add type definition file

[tienne]

This PR is associated with issue #26
added a type definition file to enable @ egjs / agent in typescript.

[coveralls]

Coverage remained the same at 93.525% when pulling 71fb953 on tienne:feature/type_definition into 9169cf7 on naver:master.

[jongmoon]

@tienne Thank you!! For your PR. We are very appreciated to you!
Could you apply our review?

[jongmoon]

@tienne Maybe you missed it that I suggested following.

export default Agent;

instead of

export = Agent;

Could be possible?

[jongmoon]

@tienne Thank you. I'll merge it!

[tienne]

@jongmoon
Can you tell when the npm distribution will go into the merged version?
Thank you

[jongmoon]

@tienne We will distribute the merged version after #29 is done. (It would be done within 2 days)

[jongmoon]

@tienne It's distributed~

[tienne]

@jongmoon Thank you for letting me know.