[polygon] Improve get function to return geometry of any region

[zerefwayne]

Returns the MULTIPOLYGON data for the requested region. Input: ISO3 Country Code, and further subdivisions

Examples:

1. get("IND") - Returns polygon of India
2. get("IND", "Uttar Pradesh") - Returns polygon of the state Uttar Pradesh
3. get("IND", "Uttar Pradesh", "Lucknow") - Returns polygon of district Lucknow

Checks:

• [x] Improve get command
• [x] Change description of command
• [x] Write tests

[zerefwayne]

Example

[codecov-io]

Codecov Report

Merging #15 into master will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #15   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines           31        45   +14     
=========================================
+ Hits            31        45   +14     

Impacted Files	Coverage Δ
src/polygon.jl	100.00% <100.00%> (ø)

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