Add custom variable tracking

[aeirola]

Should support both event and visit scopes, with matching APIs. Event scoped variables should be set with the actual event, not with separate API calls.

Event scope (need to check which all events support custom variables)

trackScreen(
  path: string,
  customVariables?: Array<{
    index: number,
    name: string,
    value: string,
  }>,
)

Visit scope

setCustomVariable(index: number, name: string, value: string)

See

• https://developers.piwik.pro/en/latest/tracker/tracker_object.html#setCustomVariable
• https://developers.piwik.pro/en/latest/sdk/Piwik_PRO_SDK_for_Android.html#tracking-custom-variables
• https://developers.piwik.pro/en/latest/sdk/Piwik_PRO_SDK_for_iOS.html#tracking-with-custom-variables