Update to address stacktrace and export_all warnings

[carlosedp]

Hope this is still maintained... :)

The PR prevents some warnings on latest rebar3 config. Works fine with Erlang 25.
Also updated the readme with some instructions (and markdown cleanups).

Previously, using eflame printed the following warnings:

_build/default/lib/eflame/src/eflame2.erl:12:2: Warning: export_all flag enabled - all functions will be exported
_build/default/lib/eflame/src/eflame2.erl:177:65: Warning: erlang:get_stacktrace/0 is removed; use the new try/catch syntax for retrieving the stack backtrace
_build/default/lib/eflame/src/eflame2.erl:198:65: Warning: erlang:get_stacktrace/0 is removed; use the new try/catch syntax for retrieving the stack backtrace
_build/default/lib/eflame/src/eflame2.erl:243:65: Warning: erlang:get_stacktrace/0 is removed; use the new try/catch syntax for retrieving the stack backtrace
_build/default/lib/eflame/src/eflame2.erl:258:65: Warning: erlang:get_stacktrace/0 is removed; use the new try/catch syntax for retrieving the stack backtrace
`