ENH: keep the common columns from either the left/right/both dfs in pd.merge.
#### Is your feature request related to a problem?

I wish I could use pd.merge and keep the common columns just from either the left/right df instead of getting col_x/col_y.

#### Describe the solution you'd like

pd.merge should get a new parameter like - keep_common_columns_from - which can take three arguments - 

1. only_left
2. only_right
3. both
