* Currently `m2cgen` generates a module in various languages that has a
  "score"/"Score" function/method. This is not always desirable, as many
  of the trained modules that are to be exported may provide their
  prediction via API functions with different names (such as `predict`).

* This commit adds a way of specifying the name of the function both via
  the CLI and in the exporters (that is, in the `export_to_` funcitons)
  by specifying the `function_name` option/parameter while keeping the
  default set to "score"/"Score" for backwards compatilibity.

Signed-off-by: mr.Shu <mr@shu.io>

* Add `function_name` to mock CLI args to be used in the tests.

Signed-off-by: mr.Shu <mr@shu.io>

* Add a test for the `--function_name` CLI option.

Signed-off-by: mr.Shu <mr@shu.io>

* Make sure function_name is actually used in the interpreters.

Signed-off-by: mr.Shu <mr@shu.io>

* Make sure that the `function_name` docstring ends with a dot rather
  than the default value.

* Add dot to various CLI argument's help messages.

Signed-off-by: mr.Shu <mr@shu.io>

* Make sure the default function_name (when not provided via arguments)
  gets backpropagated from the actual function definitions (in other
  words, make sure that the keyword arguments are respected when the
  function_name is not specified via CLI).

Signed-off-by: mr.Shu <mr@shu.io>

* Add a comment on function_name being conditionally set later.

Signed-off-by: mr.Shu <mr@shu.io>