Pass current file name to TermExec

[lunw1024]

As in #12, I'm trying to pass the current file name to TermExec.
However after the syntax update, TermExec cmd='a'.expand('%').'b' no longer works, it only considers a as the command.

[akinsho]

@IDl0T can you try #36 I implemented the functionality suggested in #12 to replace the broken syntax, so that user can now do cat % and have that expand to the current filename

[lunw1024]

It does work! Thanks for such a quick fix.

[lunw1024]

However, when I try to run TermExec cmd='g++ -std=c++17 % -o run', the command got truncated to g++ -st.
Seems like everything after -st are truncated.