ENH: A keep_fields type argument for merge operation
#### Is your feature request related to a problem?

It would increase user convenience if there were an argument where the user could specify the fields from the two data frames to be be merged which should be kept(or alternatively, a discard_fields column to specify which ones should be discarded)

This would make the merge operation more analogous to its SQL counterpart

#### Describe the solution you'd like

For two data frames A and B where A has fields 'c' and 'd' and B has fields 'e' and 'f' respectively,
it would simply result in the merge API looking like:

`pd.merge(A,B,left_on = 'c', right_on = 'e', keep_fields = ['c','d','e'])
`
