Add option to pass key without begin/end boundaries

[enver]

After merging fix for #131 a brake change has been introduced as users can't import keys without begin/end boundaries anymore. Previously boundaries were removed if they existed now beside removing boundaries there is also check which is raising error if they don't exist.

<<< error >>>

(node:1) UnhandledPromiseRejectionWarning: Error: Missing BEGIN line
    at Object.module.exports.trimSurroundingText (/app/node_modules/node-rsa/src/utils.js:96:15)
    at Object.privateImport (/app/node_modules/node-rsa/src/formats/pkcs1.js:55:33)
    at Object.detectAndImport (/app/node_modules/node-rsa/src/formats/formats.js:63:48)
    at NodeRSA.module.exports.NodeRSA.importKey (/app/node_modules/node-rsa/src/NodeRSA.js:183:22)

There should be an option to disable boundary checks https://github.com/garethsime/node-rsa/blob/4fce44453452855033f1fab8c7afbaebfcd019d9/src/formats/pkcs1.js#L17 and https://github.com/garethsime/node-rsa/blob/4fce44453452855033f1fab8c7afbaebfcd019d9/src/formats/pkcs1.js#L22 in order to provide some kind of backward compatibility or this check should be removed completely.

[rzcoder]

PR welkome

[rzcoder]

Merged