Support base64 path encoding

[1574242600]

Supports base64 path encoding to reduce filename lengths

https://rclone.org/crypt/#file-name-encryption-modes

An experimental advanced option filename_encoding is now provided to address this problem to a certain degree. For cloud storage systems with case sensitive file names (e.g. Google Drive), base64 can be used to reduce file name length. For cloud storage systems using UTF-16 to store file names internally (e.g. OneDrive, Dropbox, Box), base32768 can be used to drastically reduce file name length.