Added disable-previous-line.

[nventuro]

Similar to solhint-disable-next-line, previous-line disables linter errors on the line preceding the comment directive.

This is very useful when disabling no-empty-blocks errors, since it lets you put the directive inside the (empty) block, and not before it, where there may be comments (e.g. if it is a function).

Some examples from OpenZeppelin:

/**
 * @dev Validation of an executed purchase. Observe state and use revert statements to undo rollback when valid
 * conditions are not met.
 * @param beneficiary Address performing the token purchase
 * @param weiAmount Value in wei involved in the purchase
 */
function _postValidatePurchase(address beneficiary, uint256 weiAmount) internal view {
    // solhint-disable-previous-line no-empty-blocks
}

/**
 * @dev This contract should only be used by extending from it, deploying it as a base contract is disabled.
 */
constructor () internal {
  // solhint-disable-previous-line no-empty-blocks
} 

[coveralls]

Coverage increased (+0.002%) to 99.529% when pulling 99efbf9 on nventuro:feat/disable-previous-line into f389547 on protofire:master.

[fvictorio]

Looks good. I added an extra test for using this rule with a multi-line comment.

[nventuro]

@fvictorio note that these tests are affected by #92: we may want to merge that one first and then fix the asserts.

[nventuro]

Merged #92 into this, should be ready now.