Add snmp_syntax option

See Issue #29 and documentation at:

https://github.com/Feuerlabs/exometer/blob/uw-snmp-syntax-option/doc/exometer.md#new-3

`Options` will be passed to the entry, but the framework will recognize the following options:
...
* `{snmp, [{DataPoint, ReportInterval}]}` - defines a link to SNMP reporting, where the given data points are sampled at the given intervals, converted to SNMP PDUs and transmitted via the `exometer_report_snmp` reporter.

* `{snmp_syntax, [{DataPoint | {default}, SYNTAX}]}` - specifies a custom SNMP type for a given data point. `SYNTAX` needs to be a binary or a string, and corresponds to the SYNTAX definition in the generated SNMP MIB.

For example, the default value for an exometer counter is `"Counter32"`, which expands to `SYNTAX Counter32` in the corresponding MIB object definition. If a 64-bit counter (not supported by SNMPv1) is desired instead, the option `{snmp_syntax, [{value, "Counter64"}]}` can be added to the counter entry (note that `value` in this case is the name of the data point representing the counter value).