Adding nameparser configuration entry.

[nicolasnoble]

When embedding Lua into binary, it isn't uncommon to also package Lua files into the final product. There will usually be a custom loader to read the source files from the packaged files.

In order to maintain functionality of luacov in such circumstances, this PR provides a mechanism for the user to provide custom mappings between internal names and an external source location.

For example, if the custom file loader prefixes its files with src:, we can do something like the following:

configuration = {
  nameparser = function(name)
    if name:sub(1, 4) == 'src:' then
      return '/location/of/debug/files/' .. name:sub(5)
    end
  end
}

[hishamhm]

The description here doesn't really match the behavior in the code below:

• in the code below, the @-stripping parser still takes precedence over the user's custom nameparser whenever a name starts with @, meaning that it is not only a "default" behavior that can be overridden with nameparser, but instead nameparser is just a fallback
• the slash-conversion behavior happens after the namesparser as well.

I think a less surprising behavior would be:

• priority order to be that the nameparser takes precedence over the @-stripping (that is, match what's implied in the docs when it calls the @-stripping the "default")
• slash-conversion can remain after nameparser entries for simplicity (that is, match what is in the code), but that needs to be documented

[hishamhm]

@nicolasnoble Thank you for the PR! Left you a review comment.