Add support for any type of headers that need to be included in requests to the Prometheus API

[harryfallows]

During the resolution to issue #14, the idea was floated that instead of just supporting Authorization headers, the exporter should support all types of headers for calling the Prometheus API. So, as opposed to key-value pairs of Prometheus instance identifiers and Authorization header values, the input file would consist of identifiers mapping to many header key-value pairs.

Example:

• Current state of input file:

<prometheus-instance>: <authorization-header-value>
<another-prometheus-instance>: <authorization-header-value>

• Proposed state of input file:

<prometheus-instance>: 
        Authorization: <authorization-header-value> 
        <another-header>: <another-value>
<another-prometheus-instance>:
        Authorization: <authorization-header-value> 
        <another-header>: <another-value>
        <another-header>: <another-value>

We decided that this would not be implemented at this moment, however, if anyone were to require it we would reconsider. Please comment below if this is something you require!