Provide a config generator

An outline
```
#include
#include

// Create a config template with capitalized VARIABLE_NAMES as placeholders for
// config parameter values
std::string conf_template = R"(
num_devices = NUM_DEVICES;
num_targets = NUM_TARGETS;
capacities_mb = CAPACITIES;
block_sizes_kb = BLOCK_SIZES;
num_slabs = NUM_SLABS;
slab_unit_sizes = {
SLAB_UNIT_SIZES
};
desired_slab_percentages = {
SLAB_PERCENTAGES
};

etc ...

)";

int main() {

// Create a default Config
hermes::Config conf = hermes::InitDefaultConfig();

// Parse command line options and overwrite default Config values with the
// user-supplied values.

// Replace variables in the config template with the values in conf
conf_template = std::regex_replace(conf_template, std::regex("NUM_DEVICES"),
std::to_string(conf.num_devices));
conf_template = std::regex_replace(conf_template, std::regex("NUM_TARGETS"),
std::to_string(conf.num_targets));
// etc.

// Finally, write the final conf_template string to a file. The filename
// should be passed via command line option, with ./hermes.conf as the default

return 0;
}
```

Start from the issue's proposed config template and the hermes::Config values returned by InitDefaultConfig(). Determine how command-line options should override those values and how the output filename should be selected, including the default ./hermes.conf. Done means a generated configuration file contains the populated template and can be written to the requested path.

Written by the indexing model from the issue text.