Allow use behind proxy

[rlogiacco]

To allow setting a proxy I've added the use of the agent option in the following two sections of code

telegraf-wit/lib/wit.js

Lines 23 to 29 in 9453427

	return fetch(url, {
	method: 'GET',
	headers: {
	'Authorization': `Bearer ${this.token}`,
	'Accept': 'application/json',
	'v': this.apiVersion
	}

telegraf-wit/lib/wit.js

Lines 40 to 47 in 9453427

	return fetch(url, {
	method: 'POST',
	headers: {
	'Authorization': `Bearer ${this.token}`,
	'Accept': 'application/json',
	'Content-Type': 'application/json',
	'v': this.apiVersion
	},

They now look like

return fetch(url, {
      method: 'GET',
      agent: this.opts.agent,
      headers: {
        'Authorization': `Bearer ${this.token}`,
        'Accept': 'application/json',
        'v': this.apiVersion
      }