By Tom Westbrook SINGAPORE (Reuters) -Bitcoin neared a price level on Tuesday that could force software firm MicroStrategy Inc to add more tokens against a bitcoin-backed loan or trigger selling some of its vast holdings, setting fragile cryptocurrency markets on edge. MictroStrategy, an aggressive investor in bitcoin, said it borrowed $205 million from crypto bank Silvergate Capital in March, with the three-year loan mostly secured against some 19,466 bitcoins. If the bitcoin price dropped below about $21,000 that would trigger a “margin call” or a demand for extra capital, MicroStrategy Pres…