Add slot positions from roster settings

[rob-sumer]

In the context of a draft, it's useful to know how many slots are available for each position. Within the API, this information is available as part of the mSettings view:

// psuedo-JSON response
{
  .... 
  rosterSettings: {
    ... 
    lineupSlotCounts: {
        "0": 1, // maps to "QB" slot 
        "1": 0, // maps to "TQB" slot
        "2": 2, // maps to "RB" slot
        ... etc, goes all the way to slot 24
    }
 }
}

We can map each lineupSlotCount(s) to it's respective position and provide this information to the consumer:

from espn_api.football import League

league = League(...)
print(league.settings.position_slot_counts)

# outputs the following dict
# {'QB': 1, 'TQB': 0, 'RB': 2, 'RB/WR': 0, 'WR': 2, 'WR/TE': 0, 'TE': 1, 'OP': 0, 'DT': 0, 'DE': 0, 'LB': 0, 'DL': 0, 'CB': 0, 'S': 0, 'DB': 0, 'DP': 0, 'D/ST': 1, 'K': 1, 'P': 0, 'HC': 0, 'BE': 7, 'IR': 1, '': 0, 'RB/WR/TE': 1, 'ER': 0}

[cwendt94]

Thanks for the PR and contributing! Looking at it now!