Meeting statistics (single and multiple meeting counts)

[AaronnSeah]

part of #134 and #135
Command word: meeting stats

meeting stats [NUMBEROFMONTHS] [m/MONTHS y/YEAR]

[NUMBEROFMONTHS] is optional, only allowed values are [2,6].
[m/MONTHS y/YEAR] is optional, only allowed integer values for months are [1,12] and allowed integer values for year is [1, infinity]

only one optional parameter allowed.

3 kinds of commands:
1) get number of meetings started in the current month and year. (command: meeting stats)

2) get number of meetings started in the specified month and year. (command: meeting stats m/10 y/2020)

3) output multiple number of meetings started for months in a bar chart. (command: meeting stats 4)
for meeting stats 4, output meetings count for the months between the current month and year and the previous 3 months inclusive.

[Asthenosphere]

Tested and works well! LGTM!