Make it easier to create cli arguments

[angelosilvestre]

Currently, we need to know the syntax of ffmpeg cli arguments. For example, if we want to trim the start of an input, we need to create an argument with the following code:

CliArg(name: 'ss', value: myValueInSeconds);

Ideally, users of this package shouldn't need to know the exact syntax of a cli argument. We should abstract the underlying syntax by providing objects for the arguments. For example, for trimming the start of an input the user should be able to write a code like the following:

TrimStart(const Duration(seconds: 5));

If we want to improve the discoverability of the arguments we could use factory methods like the following:

CliArgs.trimStart(const Duration(seconds: 5));

Using factory methods the users would be able to discover the arguments by using the IDE code completion.

[angelosilvestre]

cc @matthew-carroll