Add password input from environment variable and file

[gitseti]

Motivation
resolve #103

Most of the time it is not a best practice to read in passwords in the command line in clear text like mqtt> con -i myClient -u gitseti -pw mypassword. Therefore, it is already possible to omit the password text so that the cli will prompt the user to enter the password in a masked input field.

Nevertheless, this input field shows some limitations (like the input being kept to a specific internal buffer size) so that another option for securely reading in a password is needed.

Therefore, this PR introduces two new sources for password input:

• Environment variable
• File

These options can be specified in con, mqtt pub and mqtt sub as follows:

$ export PASSWORD mypassword
mqtt> con -i myClient -u gitseti -pw:env PASSWORD

$ echo mypassword > password.txt
mqtt> con -i myClient -u gitseti -pw:file password.txt

Changes

• Refactor file conversion into an own class
• Add converters for converting a file and an environment variable to a ByteBuffer

[SgtSilvio]

Looks good, only remark is the file to string to bytebuffer conversion