Add command line argument for passing in adapter

[nileshpatra]

Currently, ADAPTERS path path is hard-coded and users need to manually modify this variable if the file is placed elsewhere.
It might be a better idea if users could pass-in file path via command line argument --adapters
If no such command line argument is passed, it'd default to the original implementation i.e. finding it in <location of binary>/../db/trimmomatic.fa (the original implementation)

This is an attempt to implement the functionality and I've tested this.

[nileshpatra]

@tseemann could you please review this?

[nileshpatra]

@tseemann gentle ping, Could you please review this?

[nileshpatra]

Gentle ping on this?

CC: @tseemann @andersgs

[nileshpatra]

@tseemann @andersgs ping again :-)

Full context: I maintain this package in debian. And for people only be able to use the package, by 'manually' editing /usr/bin/shovill in the current setting.
Hence, it'd be nice if you could merge this please.